Abstract: Equatorial/apical bond repartitioning in TBP clusters, formed by main group elements only (e.g., B5H5 2�, B3H3N2, etc.) or mixed with metal atoms (e.g., [(L2M)3S2] n , [(L3M)3S2] n , [(CpM)3S2] n ), is critically analyzed from the horizontal comparison of the experimental structures and in terms of basic MO concepts. The ideas are double-checked through specific DFT calculations or existing ab initio results. Based on the Wade�s rules for closo-clusters, the five vertices systems are normally characterized by six skeletal electron pairs. In the main group clusters, the electron delocalization at the equatorial edges depends on the electronegativity of the apical groups and for the Beq�Beq bonds an inverse relationship between bond strength and bond length is remarked. In [(L2M)3S2] n compounds with a total electron count (TEC) of 48, the six bonding electron pairs localize at the apical M�S bonds. In [(L3M)3S2] n or [(CpM)3S2] n , also with TEC = 48, the effective atomic number rule predicts three single M�M single bonds besides the six M�S apical ones. In actuality, only partial M�M bonding can be considered due to the intermediation of the capping sulphur atoms, that help shifting the antibonding character of populated radial levels with that of the vacant tangential ones (bonding). The qualitative MO arguments are supported by the topological nature of the calculated DFT wave functions. Moreover, the MO nature of three lowest LUMOs for 48e� species help to rationalize experimental structural trends observed for the addition of up to five electrons.

Abstract: We document here a spirited debate among three colleagues and friends who have strong opinions on a specific bonding problem, the presence or absence of a cross-ring sulfur�sulfur bond in a trinuclear Cu3S2 cluster. The example may seem esoteric, but through their struggles with this specific bond (and with each other) the authors approach the more general problematic of chemistry, the chemical bond. The discussion focuses on bond lengths and the population of bonding and antibonding orbitals, and on oxidation states, electron counting, and associated geometries. It expands to encompass other bonding criteria, and introduces examples ranging far across organic and inorganic chemistry. The authors suggest molecules that might test their ideas. An Appendix to the paper discusses a matter rarely broached in the chemical literature�should one review for publication a paper which criticizes one of your own contributions

Abstract: The comparison of the very similar compounds (Ph3P)2Pt(μ-S)2Pt(PPh3)2 (1) and (Ph2PyP)2Pt(μ-S)2Pt(PPh2Py)2 (2) raises intriguing questions about the reliability of the reported Pt2S2 core in 1, where the Pt-S bonds are the shortest ever reported. Also, the trans-annular S · · · S separation of 2.69 � is surprisingly shorter in 1 than in 2 (3.01 �), but no incipient coupling between two S2- bridges seems reasonable in this case. Various considerations lead to reformulate 1 as [(Ph3P)2Pt(μ-H)2Pt(PPh3)2](BF4)2, 3. The sets of cell parameters for 1 and 3 are not equal but two axes match, and the volume of 1 is exactly double. Simple matrices may be constructed to interconvert the direct and reciprocal crystalline cells, thus corroborating their identity of the compounds. It is concluded that, in the structure solution of 1, some atoms were either neglected (BF4 - counterions) or ill identified (sulfido in place of hydroxo bridges), while the structure of 3 was solved by collecting only one-half of the possible reflections (hence, also the different space groups). A new preparation, crystallization and X-ray structure of 3 confirms the above points and dismisses any other theoretical conjecture about two electronically different Pt2S2 cores in 1 and 2.

Abstract: The bonding of a rare S4 2- rectangle coordinated to four transition metals (synthesized by Isobe, Nishioka, and co-workers), [{M2(h5-C5Me5)2(m-CH2)2}2(m-S4)]2 (M=Rh, Ir) is analyzed. DFT calculations indicate that, while experiment gives the rectangle coordinated with its long edge parallel to Rh-h bonds and perpendicular to the Ir-Ir bonds, either orientation is feasible for both metals. Although rotation of the S4 rectangle is likely a multi-step process, a calculated barrier of 46 kcalmol-1 for a simple interconversion pathway goes through a trapezoidal, not a square, transition state An argument is presented, based on molecular orbital (MO) calculations, that the long S-S contacts (2.70 and 2.90 A) in the rectangle are in fact twocenter, three-electron bonds (or �halfbonds�). Moreover, the 2- charge on the S4 rectangle is related to a Jahn�Teller distortion from a square to a rectangle. Finally, DFT is used to explore possible stable intermediates in the oxidative process giving these M4S4 2 compounds: for Ir, the coupling of two Ir2S2 molecules appears feasible, as opposed to a possible two-electron oxidation of a neutral Rh4S4 molecule.

Abstract: We recently studied an elongated S-4(2-) rectangle bridging transition metals such as Ir, Rh, Cu, or Fe and found that its formation is related to the oxidation of parallel S-2(2-) molecules. The removal of two electrons can occur either externally or by an internal metal-ligand redox process that implies depopulation of the high-lying S-4 sigma(*) MO and population of a lower metal level. Thus, the interaction may be described as metal back-donation. However, sufficiently electropositive metals can more easily reach higher oxidation states and this reverses the direction of the two-electron interaction. In such a case, the system may be viewed as formed by two uncoupled disulfides donating an additional electron pair to the metals. In this paper, we analyze the aspects of the 2S(2)(2-)/S-4(2-) dichotomy through DFT calculations and qualitative MO analysis of the known bimetallic MS4M cores having either triple-decker (TD) or chair-type structures. We correlate the extent of the disulfide coupling with the nature (electronegativity) of the terminal metal fragments (the metals range from Group IV to X) beside their electron configuration. We find that the formation of an S-4(2-) unit is favored by metals which cannot stabilize high oxidation states (e. g. Fe(IV) or Cu(III)), whereas the two S-2(2-) units remain substantially uncoupled with early transition metal in high oxidation states (e.g. Ti(IV)). (c) 2008 Elsevier B.V. All rights reserved.

Abstract: Solution multinuclear and multidimensional NMR analyses of the [(triphos)Rh(eta(1):eta(2)-P4RR')](n+) complex cations [triphos = MeC(CH2PPh2)(3); R = H, Me, Ph; R' = lone pair, H, Me; n = 0, 1] confirm the same primary structure determined by X-rays in the solid state. In addition, 2D H-1 NOESY and P-31{H-1} exchange NMR spectroscopy show that these complexes are nonrigid on the NMR time-scale over the 253-318 K temperature range. A dynamic process that involves the terminal phosphane groups of the triphos ligand is displayed by each compound. The NMR spectroscopic data indicate a slow scrambling motion in which the P4R unit tumbles with respect to the (triplios)Rh moiety. DFT calculations outline a possible turnstile mechanism involving the threefold and twofold rotors into which the complex is subdivided. The process goes through a transition state in which the axial and equatorial dispositions of the PRR' and P=P donating groups of the P4RR' ligand are inverted with respect to the ground state. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim., Germany, 2008)

Abstract:

Abstract: A series of cobalt complexes containing the dianionic ligands X(CH,COO)(2) 2- (X = S, tda; or X = O, oda) is reported. The complexes [Co(tda)(bipy)(H2O)center dot-4H(2)O (2), which was characterised by X-ray crystallography, [Co(tda)(phen)(H2O)] (3), [Co(tda){(MeO)(2)bipy}(H2O)]-2H(2)O (4) and [Co(tda)(dpp)(H2O)]center dot 2H(2)O (5) are obtained by reaction of the precursor complex [Co(tda)(H2O)], (1a) with the bidentate N-donor ligands 2,2'-bipyridine (bipy), o-phenanthroline (phen), 4,4'dimethoxy-2,2'-bipyridine [{(MeO)2bipy] and 2,3-bis(2-pyridyl)pyrazine (dpp), respectively. The reaction of la with di(2-pyridyl) ketone unexpectedly displaces tda from the coordination sphere and leads to the Co-III species [Co(OC(OH)(2-pyridyl)2-puridil)(2)}(2)](+)[Htda]center dot 3H(2)O (6), as confirmed by an X-ray analysis. Three new oxydiacetate complexes of cobalt are also synthesised by treating the known complex [(Co(oda)(H2O)2)-H2O], with 4,4'-di-tert-butyl-2,2'-bipyridine (tBu(2)bipy), 2,3-bis(2-pyridyl)pyrazine (dpp) and 4,4'-bipyridine (4,4'-bipy). The complexes [Co(oda)(tBu(2)bipy)(H2O)]center dot l.5H(2)O (7), [Co(oda)(dpp)(H2O)j-2H(2)O (8) and [Co(oda)(H2O)2)2(P4,4'-bipy)]-2H(2)O (9) are structurally characterised (8 as a methanol solvate). Complex 9 is binuclear, in contrast to 7 and 8 (mononuclear), and magnetic susceptibility measurements down to 2 K for this complex show high-spin noncorrelated Con ions with a typical Curie-Weiss behaviour. The Co(oda) fragment in 7-9 adopts a planar (mer) rather than a folded (fac) conformation, the latter of which is common to all the M(tda) complexes but is only occasionally observed in the M(oda) ones. In the final section of the paper, the key geometric parameters that control the stereochemistry of the two ligands are determined from a statistical analysis based on the structurally characterised Co(oda) and Co(tda) complexes. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).

Abstract: Ab initio molecular dynamics (AIMD) calculations, based on the Car-Parrinello method, have been carried out for three models of heme c that is present in cytochrome c. Both the reduced (Fe(II)) and oxidized (Fe(III)) forms have been analyzed. The simplest models (1R and 1O, respectively) consist of a unsubstituted porphyrin (with no side chains) and two axially coordinated imidazole and ethylmethylthioether ligands. Density functional theory optimizations of these models confirm the basic electronic features and are the starting point for building more complex derivatives. AIMD simulations were performed after reaching the thermal stability at T = 300 K. The evolution of the Fe-L-ax bond strengths is examined together with the relative rotations of the imidazole and methionine about the axial vector, which appear rather independent from each other. The next models (2R and 2O) contain side chains at the heme to better simulate the actual active site. It is observed that two adjacent propionate groups induce some important effects. The axial Fe-S delta bond is only weakened in 2R but is definitely cleaved in the oxidized species 2O. Also the mobility of the Im ligand seems to be reduced by the formation of a strong hydrogen bond that involves the Im N delta 1-H delta 1 bond and one carboxylate group. In 2O the interaction becomes so strong that a proton transfer occurs and the propionic acid is formed. Finally, the models 3 include a free N-methyl-acetamide molecule to mimic a portion of the protein backbone. This influences the orientation of carboxylate groups and limits the amount of their hydrogen bonding with the Im ligand. Residual electrostatic interactions are maintained, which are still able to modulate the dissociation of the methionine from the heme.

Abstract: The position of the sulfur atom in the thienyl group of 6-(thienyl)-2-(imino)pyridine ligands strongly affects the catalytic activity of the corresponding tetrahedral high-spin dihalide Co-II complexes in the oligomerization of ethylene to alpha-olefins upon activation with methylaluminoxane (MAO). Complexes with the sulfur atom in the 3-position of the thienyl ring catalyze the selective conversion of ethylene to 1-butene, while catalysts containing thien-2-yl groups give C-4-C-14 alpha-olefins. In situ EPR experiments showed the occurrence of a spin state changeover with the formation of low-spin Co-II species upon activation of the catalyst precursors by MAO. DFT calculations suggest that only thien-2-yl rings allow for the coordination of the sulfur atom to the cobalt center in the MAO-activated systems.

Abstract: The alpha-diimino chelating ligand 1,4-diaza-1,3-butadiene, or its dianionic enediamido form (generically defined as DAD), forms a variety of complexes with group 4 transition metals either with or without other co-ligands. The known species are either pseudo -octahedral [L4M(DAD)] or tetracoordinate [L2M(DAD)] with a lower degree of metal saturation in the latter case. Additionally, there are a number of known CpMLn species and homoleptic complexes with the formula [M(DAD)(3)] or [M(DAD)(2)]. The M-DAD metallacycle is generally either planar or folded at the (NN)-N-... vector, the latter arrangement being observed when the number of co-ligands is reduced and hence there is insufficient saturation at the metal. The conformation also depends on the relative electron population of DAD, which is a noninnocent ligand and exists as a continuum between the neutral and dianionic resonance forms. Herein some theoretical concepts, that have previously been developed to describe the bonding capabilities of DAD toward different triads of transition metals, are extended to complexes of the group 4 metals. New electronic aspects of the [M(DAD)(3)] complexes are described together with a completely new analysis of the [M(DAD)(2)] complexes. These exhibit the planar/bent conformational dichotomy, which may have an electronic and/or steric origin as it is affected by the type of substituent at the nitrogen atoms. A reasonable justification of the observed conformational preferences is found from a combination of density functional and QTAIM analyses. The latter clearly demonstrates that a peculiar intramolecular hydrogen bonding can contribute to the stabilization of planar-type structures, ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).

Abstract:

Abstract: Reaction of a variety of CCH bond-containing 1,6-enynes with N2CHSiMe3 in the presence of RuCl(COD)Cp* as catalyst precursor leads, at room temperature, to the general formation of alkenylbicyclo[3.1.0]hexanes with high Z-stereoselectivity of the alkenyl group and cis arrangement of the alkenyl group and an initial double-bond substituent, for an E-configuration of this double bond. The stereochemistry is established by determining the X-ray structures of three bicyclic products. The same reaction with 1,6-enynes bearing an R substituent on the C-1 carbon of the triple bond results in either cyclopropanation of the double bond with bulky R groups (SiMe3, Ph) or formation of alkylidene-alkenyl five-membered heterocycles, resulting from a beta elimination process, with less bulky R groups (R = Me, CH2CHCH2). The reaction can be applied to in situ desilylation in methanol and direct formation of vinylbicyclo[3.1.0]hexanes and to the formation of some alkenylbicyclo[4.1.0]heptanes from 1,7-enynes. The catalytic formation of alkenylbicyclo[3.1.0]hexanes also takes place with enynes and N2CHCO2Et or N2CHPh. The reaction can be understood to proceed by an initial [2+2] addition of the RuCHSiMe3 bond with the enyne CCH bond, successively leading to an alkenylruthenium-carbene and a key alkenyl bicyclic ruthenacyclobutane, which promotes the cyclopropanation, rather than metathesis, into bicyclo[3.1.0]hexanes. Density functional theory calculations performed starting from the model system Ru(HCCH)(CH2CH2)Cl(C5H5) show that the transformation into a ruthenacyclobutane intermediate occurs with a temporary eta(3)-coordination of the cyclopentadienyl ligand. This step is followed by coordination of the alkenyl group, which leads to a mixed alkyl-allyl ligand. Because of the non-equivalence of the terminal allylic carbon atoms, their coupling favors cyclopropanation rather than the expected metathesis process. A direct comparison of the energy profiles with respect to those involving the Grubbs catalyst is presented, showing that cyclopropanation is favored with respect to enyne metathesis.

Abstract: The article illustrates the advantages of partitioning the total electron density rho(r(b)), its Laplacian del(2)rho(r(b)), and the energy density H(rb) in terms of orbital components. By calculating the contributions of the mathematically constructed molecular orbitals to the measurable electron density, it is possible to quantify the bonding or antibonding character of each MO. This strategy is exploited to review the controversial existence of direct Fe-Fe bonding in the triply bridged Fe-2(CO)(9) system. Although the bond is predicted by electron counting rules, the interaction between the two pseudo-octahedral metal centers can be repulsive because of their fully occupied t(2g) sets. Moreover, previous atoms in molecules (AIM) studies failed to show a Fe-Fe bond critical point (bcp). The present electron density orbital partitioning (EDOP) analysis shows that one sigma bonding combination of the t(29) levels is not totally overcome by the corresponding sigma* MO, which is partially delocalized over the bridging carbonyls. This suggests the existence of some, albeit weak, direct Fe-Fe bonding.

Abstract: Novel zinc thiodiacetate complex [Zn(tda)(H2O)](n) (1) (tda = S(CH2COO)(2)(2-)) was obtained from the reaction of aqueous solutions of 2 zinc dichloride with a 1: 1 mixture of Na2CO3 and thiodiacetic acid. Complex 1 is a different hydrate compound with respect to the known species [Zn(tda)(H2O)(3)] center dot H2O. The selective formation of 1 or [Zn(tda)(H2O)(3)] center dot H2O can be achieved by controlling the crystallization conditions and it is even possible to interconvert the two hydrate compounds by fast or slow crystallization processes. The reaction of 1 with o-phenanthroline (phen) gave the dinuclear compound [Zn(tda)(phen)](2) center dot 5H(2)O (2), which was identified by X-ray analysis. The water crystallization molecules are interconnected by hydrogen bonds and they are aligned inside channels, which are parallel to the c-axis. (c) 2005 Elsevier B.V. All rights reserved.

Abstract: The species Ru-3(H)(mu(3)-NPPh3)(CO)(9) occurs in the solid state as two isomers, characterized either by one capping hydride and three CO bridges (1a) or by one bridging hydride and all terminals COs (1b); key intermediates for the formation, fluxionality and solvent-dependent interconversion of the isomers are highlighted through a DFT MO analysis.

Abstract: Protonation across the metal-metal bond in the complexes [(CO)(2)M(mu-dppm)(mu-PtBu2)(mu-H)-M(CO)(2)] (M=Fe or Ru, dppm= Ph2PCH2PPh2) induces M-M bond shortening of up to about 0.05 angstrom. DFT calculations on simplified iron models reproduce this trend well. Conversely, the computations show that the M-M distance in the dimer [{Cp*Ir(CO)}(2)] lengthens with two consecutive protonations, but there are no crystal structure determinations to highlight the effects on the Ir-Ir bond. DFT calculations and the analogous cobalt system confirm that the transformation of a two-electron, two-center (2e-2c) bond into a 2e-3c bond is accompanied by the predicted elongation. An MO analysis indicated similar nature and evolution of the M-M bonding these cases. In particular, the HOMOs of the mono-hydrido cations [Cp(CO)M(mu-H)M(CO)Cp](+) (M=Ir, Co) have evident M-M bent-bond character, and hence subsequent protonation invariably causes a decrease in the bond index. The Fee and Cot systems have also been analyzed with the quantum theory of atoms. in molecules (QTAIM) method, but in no case was an M-M bond critical point located unless an artificially shorter M-M distance was imposed. However, the trends for the atoms-in-molecules (AIM) bond delocalization indexes delta(M)-(M) confirm the overall M-M bond weakening on protonation. In conclusion, all the computational results for the iron system indicate that the paradigm of a direct correlation between bond strength and distance is not always applicable. This is attributable to a very flat potential energy surface and various competing effects imposed by the bridging ligands.

Abstract: The reaction between equimolar amounts of Pt-3(mu-PBu'(2))3(H)(CO)(2), Pt3H, and CF3SO3H under CO atmosphere affords the triangular species [Pt-3(mu-PBu'(2))(3)(CO)(3)]X, [Pt-3(CO)(3)(+)]X (X = CF3SO3-), characterized by X-ray crystallography, or in an excess of acid, [Pt-6(mu-PBu'(2))(4)(CO)(6)]X-2, [Pt-6(2+)]X-2. Structural determination shows the latter to be a rare hexanuclear cluster with a Pt-4 tetrahedral core formed by joining the unbridged sides of two orthogonal Pt-3 triangles. The dication Pt-6(2+) features also extensive redox properties as it undergoes two reversible one-electron reductions to the congeners [Pt-6(muPBu'(2))(4)(CO)(6)](+) (Pt-6(+), E-1/2 = -0.27 V) and Pt-6(mu-PBu'(2))(4)(CO)(6) (Pt-6, E-1/2 = -0.54 V) and a further quasi-reversible two-electron reduction to the unstable dianion Pt-6(2-) (E-1/2 = -1.72 V). The stable radical (Pt-6(+)) and diamagnetic (Pt-6) species are also formed via chemical methods by using 1 or 2 equiv of Cp2CO, respectively; further reduction of Pt-6(2+) causes fast decomposition. The chloride derivatives [Pt-6(mu-PBu'(2))(4)(CO)(5)Cl]X, (Pt6Cl+)X, and Pt-6(mu-PBu2)(4)(CO)(4)Cl-2, Pt6Cl2, observed as side-products in some electrochemical experiments, were prepared independently. The reaction leading to Pt-3(CO)(3)(+) has been analyzed with DFT methods, and identification of key intermediates allows outlining the reaction mechanism. Moreover, calculations for the whole series Pt-6(2+) --> Pt-6(2-) afford the otherwise unknown structures of the reduced derivatives. While the primary geometry is maintained by increasing electron population, the system undergoes progressive and concerted out-of-plane rotation of the four phosphido bridges (from D-2d to D-2 symmetry). The bonding at the central Pt-4 tetrahedron of the hexanuclear clusters (an example of 4c-2e(-) inorganic tetrahedral aromaticity in Pt-6(2+)) is explained in simple MO terms.

Abstract:

Abstract: The novel cobalt thiodiacetate complex [Co(tda)(H2O)](n), (1a) (tda = S(CH2COO)(2)(2-)) was synthesized from the reaction of aqueous solutions of cobalt dichloride with a 1: 1 mixture of Na2CO3 and thiodiacetic acid. A second isomer was also obtained occasionally and identified as complex [Co(tda)(H2O)(3)] (1b). The reaction of 1a with 2,2'-bipyridine (bipy) gave compound [Co(tda)(bipy)(H2O)] • 4H(2)O (2), which has been structurally characterized and represents, to our knowledge, the first X-ray determination of a thiodiacetate-cobalt complex. The coordination is monomeric pseudo-octahedral as determined by the typical tridentate thiodiacerate ligand with a facial configuration, the chelate bipy ligand and one water molecule. Conversely, the interaction of bis(2-pyridyl)ketone with la afforded the compound [Co{OC(OH)(2-pyridyl)(2)}(2)](+)[Htda](-) • 3H(2)O (3), as confirmed from an X-ray analysis. The bis(2-pyridyl) ketone ligand is hydrolyzed with a concomitant oxidation of the metal to Co(III), a process which has been previously described, while the tda ligand, displaced from the coordination sphere, is monoprotonated and acts as counterion. © 2005 Elsevier B.V. All rights reserved.

Abstract:

Abstract: The paper presents a detailed experimental and theoretical study of the four mixed nickel-bisdithiolene complexes [Ni(Pr(2)(i)pipdt)(dmit)] (1b, Pr(2)(i)pipdt = 1,4-diisopropyl-piperazine-3,2-dithione; dmit = 1,3-dithiolo-2-tione-4,5-dithiolato), [Ni(R(2)pipdt)(mnt)] (2b", R = 2-ethylhexyl; mnt = maleonitriledithiolato), [Ni(Pr(2)(i)timdt)(dmit)] (3b, Pr(2)(i)timdt = 1,3-diisopropyl-imidazoline-2,4,5-trithione), and [Ni(Pr(2)(i)timdt)(mnt)] (4b), and their models. All the complexes, with common (C2S2)Ni(C2S2) core and two different terminal groups, are uncharged and square-planar coordinated. Previous measurements of the first molecular hyperpolarizability indicated that some of the species are potential NLO chromophores due to the pi-delocalized character of two frontier levels (HOMO and LUMO) which is asymmetrically perturbed by the combination of one push (R(2)pipdt, R(2)timdt) with one pull ligand (dmit and mnt). The X-ray structure of complex 1b is presented and its geometry is compared with those available in the literature for the four types of complexes under study. The results of electrochemical and spectroscopic measurements (oxidation and reduction potentials, IR, dipole moment, molecular absorptivities, etc.) indicate rather different responses between the pairs of complexes 1-2 and 3-4. Hence, DFT calculations on the model compounds 1a-4a, where hydrogen atoms replace the alkyl groups of R(2)pipdt and R(2)timdt, have been carried out to correlate geometries and electronic structures. Moreover, the first molecular hyperpolarizabilities have been calculated and their components have been analyzed with the simplest two-level approximation. The derived picture highlights the different roles of the two push and pull ligands, but also the peculiar perturbation of the pi-electron density induced by the terminal CS3 grouping of the ligand dmit.

Abstract: A neutron diffraction study of the complex RuCl2[PPh2(2,6-Me2C6H3)](2) (1) defines the precise nature of the delta agostic interactions between the unsaturated metal center and two o-methyl groups of the xylyl substituents. The CH3 carbon atoms lie in the RuP2 equatorial plane with Ru...C distances of 2.637(7) and 2.668(6) Angstrom, whereas four short Ru...H distances (from 2.113(11) to 2.507(11) Angstrom) indicate that each methyl group interacts with two C-H bonds. A survey of the X-ray structures with beta, gamma, delta, and epsilon M...H3C-C moieties (no neutron data have been previously reported) shows a linear correlation between the angle M...C-C and the torsion of the methyl group about the C-C bond. Thus, the agostic interactions span the range between the classical (M...eta(2)-HC) and the nonclassical (M...eta(3)-H2C) types. A solution study of 1 shows intramolecular rearrangement of each xylyl substituent that equilibrates the environments of its two ortho CH3 groups. Activation parameters, evaluated from the analysis of H-1 NMR line shape as a function of temperature, are DeltaH(double dagger) = 9.6 +/- 0.2 kcal mol(-1) with DeltaS(double dagger) = -15.4 +/- 0.7 eu (CDCl3). The related 14-electron complexes RuX2[PPh2(2,6-Me2C6H3)](2) (X = I, 2; NCO, 3), prepared from 1 and NaX, show a similar dynamic process in solution, with the iodo derivative displaying the most hindered rotation of the xylyl group. A DFT optimization of the complex RuCl2[PH2(2,6-Me2C6H3)](2) (1a) reproduces well the nonclassical Ru...eta(3)-H2C agostic mode, whereas the classical Ru...eta(2)-HC one corresponds to a transition state 1b, destabilized by 3.4 kcal mol(-1). A similar barrier (ca. 3.8 kcal mol(-1)) is calculated for the xylyl rotation in the further simplified model RuCl2[PH2(2,6-Me2C6H3)][PH2CH=CHCH3] (1c), the absence of bulky phenyl substituents; being largely responsible for the difference with respect to the experimental value. Finally, the MO analysis addresses the intrinsic stability of the 14-electron complex RuCl2(PH3)(2) and, in agostic complexes, accounts for the different interactions between the methyl group and the metal atom in relation to the length of their interconnecting chain.

Abstract: The redox capabilities of some binuclear and trinuclear ruthenium complexes with bridging o-phenylendiamido ligands, C6H4(NH)(2)-o, have been investigated. The known species [Ru-2{mu-C6H4(NH)(2)-o}(mu-dppm)(CO)(2)(PPh3)(2)] (1) and [Ru2Mo{mu-C6H4(NH)(2)-o}(2)(CO)(6)(PPh3)(2)] (5), reacted with [Fe(Cp)(2)](PF6), yield the salts [Ru-2{mu-C6H4(NH)(2)-o(mu-dppm)(CO)(2)(PPh3)(2)](PF6)(2) (2) and [Ru2Mo{mu-C6H4(NH)(2)-o}(2)(CO)(6)(PPh3)(2)](PF6)(2)(CH2Cl2)-C-. (6), respectively (dppm = Ph2PCH2PPh2). Binuclear compounds analogous to 2 were obtained with pairs of PPh3 or CO ligands in place of the diphosphine dppm. The redox properties of the compounds have been explored also with electrochemical methods. The structural determinations of 2 and 6 show, as an evident consequence of the oxidation, the bending of the originally upright C6H4(NH)(2)-o bridge toward one metal that is eventually eta(4)-coordinated by the ligand. In the trinuclear species 6, such a bending is limited to only one bridge and is directed toward the central molybdenum atom. Another structural consequence, attributable to electronic effects, is the reciprocal reorientation of two terminal L3M fragments with modes that are different in 2 and 6, respectively. These aspects have been highlighted through a theoretical analysis with DFT calculations and an extension of the perturbation theory arguments already introduced for the precursors 1 and 5. There is evidence that, in each case, the oxidation affects mainly the bridging chelate C6H4(NH)(2)-o that transforms from diamido to diimino in character. The activated back-donation from the eta(4)-coordinated metal into the diimino ligand causes also an evident weakening of the metal-metal bond. Complex 6 represents the first well-characterized case where one o-phenylenediamido and one o-diiminobenzene coexist as ligands.

Abstract: Mononuclear complexes, which contain a dianionic ene-diamido ligand bound to a group 4 metal atom in the formal do configuration, are analyzed by the DFT method to interpret the electronic origin of the folding at the five-membered 2,5-diazapent-3-ene metallacycle moiety. Geometry optimizations were carried out for the following models, TiCl2[o-(Me3SiN)(2)C6H4], Ti(OPh)(2)(DAD), CpTiCI(DAD), CpTiMe(DAD) and CP2Zr(DAD) (DAD= HNCHCHNH). They show some common electronic features, the nature of the HOMO, in particular. In all cases, the latter results from the donation of a filled pi(3)*, ene-diamido level into an empty a metal orbital, this being maximized upon the folding of the metallacycle. Such a geometric rearrangement involves the filled nitrogen p(pi) lobes, while the C=C pi bond remains essentially uninvolved. The feature is confirmed by the application of the atom in molecules (AIM) theory, that provides no evidence of critical points between the metal center and the pair of two carbon atoms. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: Compound [(Fe(oda)(H2O)(2)} . H2O](n) (1) [oda = O(CH2COO)(2)(2-)] has been obtained by reaction of FeCl2 . 4H(2)O with a 1:1 mixture of O(CH2COOH)(2) and Na2CO3 in water. The structure is polymeric with concatenated {Fe(oda)(H2O)(2)} units extended in one direction. Each iron centre is six-coordinated by the tridentate planar oda ligand, two mutually trans water molecules and one oda oxygen atom from an adjacent {Fe(oda)(H2O)(2)} unit. Complex 1 is isomorphous with cobalt and zinc analogues. Magnetic susceptibility measurements down to 2 K showed high-spin non-correlated Fe(II) ions with a typical Curie-Weiss behaviour. Differential-pulse voltammetry establishes a value of 0.488 V versus NHE for the Fe(III)/Fe(II) redox potential of this iron complex in 0.25 mol dm(-3) NaNO3. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: The aerobic aqueous solution syntheses and structures of monomeric and polymeric Mn-II complexes containing the oxydiacetate ligand [O(CH2COO)(2)(2-)= oda] are reported. The magnetic properties of the polymeric products have also been investigated. The initially obtained species [{Mn(oda)(H2O)}(H2O)-H-.](n) (1) has been reacted with bidentate or tridentate N-donor ligands such as o-phenanthroline (phen), 2,2'-bipyridine (bipy) and 2,2':6,2"-terpyridine (terpy) to give the complexes [{Mn(oda)(phen)}(.)4H(2)O](n) (2), [Mn(oda)(bipy)(H2O)](.)2H(2)O (3) and [Mn(oda)(terpy)](.)2H(2)O (4), respectively. Species 1-4 are the first Mn-oda structures determined by X-ray methods. In all cases, the oda acts as a tridentate ligand toward one metal and adopts the typical planar arrangement. However, while the 3 and 4 are monomers, 2 and I are polymers as oda exploits both atoms of its carboxylate groups to coordinate other metals; 1 consists of a three-dimensional diamond-type network, while 2 is one-dimensional, consisting of an extended chain of {Mn(oda)(phen)} subunits. For both compounds, magnetic susceptibility measurements down to 2 K showed only weak antiferromagnetic interactions between high-spin Mn-II ions. Despite the expectedly similar coordinating capabilities of N-donor ligands, the bipy complex 3 is a monomer with the six-coordination completed by one water ligand. The tridentate nature of simultaneously present oda and terpy ligands excludes the coordination of water in complex 4, the geometry of which is distorted from the octahedron, most likely due to a peculiar packing effect. Finally, the known five-coordinate complex [MnCl2(terpy)] (5), a side product of our synthetic experiments, has also been structurally characterized. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Abstract: The dicationic complex [(triphos)Rh(mu-S)(2)Rh(triphos)](2+), 1 (modeled as 1 c) [triphos = CH3C(CH2PPh2)(3)], is known to activate two dihydrogen molecules and produce the bis(mu-hydrosulfido) product [(triphos)(H)Rh(mu-SH)(2)Rh(H)(triphos)](2+), 2 (modeled as 2b), from which 1 is reversibly obtained. The possible steps of the process have been investigated with DFT calculations. It has been found that each d(6) metal ion in 1c, with local square pyramidal geometry, is able to anchor one H-2 molecule in the side-on coordination. The step is followed by heterolytic splitting of the H-H bond over one adjacent and polarized Rh-S linkage. The process may be completed before the second H2 molecule is added. Alternatively, both H-2 molecules are trapped by the Rh2S2 core before being split in two distinct steps. Since the ambiguity could not be solved by calculations, P-31 and H-1 NMR experiments, including para-hydrogen techniques, have been performed to identify the actual pathway. In no case is there experimental evidence for any Rh-(eta(2)-H-2) adduct, probably due to its very short lifetime. Conversely, 1H NMR analysis of the hydride region indicates only one reaction intermediate which corresponds to the monohydride-mu-hydrosulfide complex [(triphos)Rh(H)(mu-SH)(u-S)Rh(triphos)](2+) (3) (model 5a). This excludes the second hypothesized pathway. From an energetic viewpoint the computational results support the feasibility of the whole process. In fact, the highest energy for H-2 activation is 8.6 kcal mol(-1), while a larger but still surmountable barrier of 34.6 kcal mol(-1) is in line with the reversibility of the process.y

Abstract: Thermal reaction of white phosphorus with [(triphos)RhH3] (1) in THF affords [(triphos)Rh(eta(1):eta(2)-P4H)] (2), triphos = MeC(CH2PPh2)(3). Similar complexes [(triphos)Rh(eta(1):eta(2)-P4R)] (R = Me (7), Et (8), Ph (9)) also form at lower temperature by the reaction of P-4 and [(triphos)Rh(R)(eta(2)-C2H4)] with elimination of ethene. In contrast, a double-insertion process follows the reaction of [(triphos)Rh(H) (eta(2)-C2H4)] and P-4 to generate tetraphosphido ethyl complex 8. Compounds 2, 7, 8 and 9 are thermally unstable and eventually decompose into the cyclotriphosphorus complex [(triphos)Rh(eta(3)-P-3)] (3) plus other unidentified phosphorus-containing species. Otherwise, PH3 or PH2R is generated in the presence of H-2. The formation of PH3 and 3 is quantitative starting from the precursor 2. The electrophilic attack of MeOTf or HBF4 on the P4R ligand in the complexes 2, 7-9 is regioselective, and yields a cationic product of formula [(triphos)Rh(eta(1):eta(2)- P4RR')](+). The direct attack on the substituted p-R phosphorus atom is demonstrated by crossing experiments. Complexes of the latter type have been isolated in the solid state for the combinations R = H and R' = Me (11) or R = Ph and R' = Me (12). The latter species, [(triphos)Rh(eta(1):eta(2)-P4PhMe')]OTf.2CH(2)Cl(2) (OTf = triflate), has been characterised by X-ray methods. The geometry at the metal is better described as a trigonal bipyramidal than pseudo-octahedral. In fact, the P4RR' unit acts as a bidentate ligand with its exocyclic PR2 donor group and the endocyclic, dihapto-coordinated P=P linkage. The latter group lies in the equatorial plane, in a similar way to a classic olefin ligand that is coordinated to a butterfly-shaped L4M fragment (M = d(8)). DFT calculations on a model of 2 and all possible protonated isomers confirm that double substitution at the exocyclic P-donor positions of the open P-4 unit is energetically favoured. A multinuclear and multidimensional NMR analysis confirms that this structure is maintained in solution for both the parent and the protonated compounds.

Abstract: Known complexes of general formula LnM(diim), where diim = C6H4(NH)(2)-o, alpha-diiminobenzene, n = 3, 4 and M is a metal of Group 8 (Ru(II), in particular) are analyzed in terms of their electronic structure and associated energetics. The newly synthesized compound [(CO)(2)(PPh3)BrRu(C6H4(NH)(2)-o)]Br, 1, is a new member of the category. This is confirmed by the X-ray structure of 1 that is presented together with electrochemical measurements and other spectroscopic characterization. At variance with the isoelectronic L4M(o-phenylenediamido) compounds (M = Group 6 metal) that are affected by structural deformations, 1 and similar complexes have a regular pseudo-octahedral structure. Density functional theory calculations and qualitative MO arguments provide an explanation for the different electronic trends as well as for the diimino/diamido dichotomy of the non-innocent chelate ligand. The latter is affected, not only by the nature of the metal (energy of d orbitals), but also by the surrounding environment (three or four coligands). Moreover, the calculations allow to outline the stability trends for the loss of one coligand from L4M(diim) systems when the Ru(II) complexes are chemically or electrochemically reduced. (C) 2003 Elsevier Science B.V. All rights reserved.

Abstract: The complex [V(O)(oda)(H2O)(2)] 1 [oda = O(CH2COO-)(2)] has been obtained by reaction of aqueous solutions of [V(O)(acac)(2)] with oxydiacetic acid. The coordination geometry around the vanadium in 1 is distorted octahedral with mutually trans water ligands and a planar conformation of the oda ligand ( mer). The relative stability of the experimentally determined structure and that of the alternative isomer with a puckered oda conformation (fac) has been investigated by DFT calculations. The latter confirms that the mer geometry is more stable by 35.6 kJ mol(-1). The complex [V(O)(oda)(mu-OMe)](2)[Na(H2O)](2) 2a is obtained by reaction of 1 with NaOH in methanol. The anions feature two V( O)( oda) units symmetrically bridged by methoxy groups in a syn-orthogonal arrangement, while the oda ligands have the fac conformation. Similar complexes [V(O)(oda)(mu-OMe)](2)[K(H2O)](2) 2b and [V(O)(oda)-(mu-OMe)](2)[HL](2) 2c-2g are obtained from reaction with KOH or with selected N-donor bases ( L), respectively. An excess of pyridine, however, causes the simple substitution of the water molecules in 1 and generates the monomer [V(O)(oda)(py)2](3). Antiferromagnetic behaviour of binuclear compounds 2a, 2d and 2e is suggested by a variable-temperature study. Finally, the reaction of 1 with bidentate N-donor ligands allows the synthesis of the complexes [V(O)(oda)(N-N)].H2O (N-N = 2,2'-bipyridine, 4; o-phenanthroline, 5). X-Ray characterization of 4 shows that the conformation of oda has changed from mer (as in the reactant 1) to fac.

Abstract: The compound [Mn(tda)(bipy)] (tda = S(CH2COO)(2)(2-)) features the first structurally characterized tetra-carboxylate dimanganese antiferromagnetic system with 'copper acetate' core; the binuclear units, completed by bipyridine chelates, are doubly chained by the tda anions that adopt an unprecedented coordination mode.

Abstract: The SiMe3 group (TMS), introduced as a substituent at the cyclopentadienyl ligand, is found to magnify the solubility of the corresponding metal complexes in supercritical carbon dioxide (scCO(2)). This is verified from comparative solubility measurements of the species (eta(5)-Me-3 SiC5H4)MoO2 Cl, 1a, (eta(5)-Me3SiC5H4)(2)ZrCl2, 2a, and (eta(5)-Me3SiC5H4)Co(CO)I-2.0.5(I-2), 3a (newly synthesised), and of their unsubstituted precursors 1b-3b, respectively. In spite of the increased solubility, the chemical, structural and reactivity properties of the TMS derivatives are scarcely affected. Confirmation comes from a detailed study of the cobalt complex 3a that includes X-ray structural determination. The geometry is most similar to that of the precursor 3b while an apparently different Co-CO interaction is observed in the carboxylated analogue [(eta(5)-PhCH2CO2C5H4)Co(CO) I-2, 3c]. The problem is computationally tackled by using the DFT B3LYP method. The optimised geometries of the simplified models of 3a-3c are all very similar. In particular, the computed stretching frequency of the unique CO ligand is consistent with the insignificant influence of the TMS group while it suggests a reduced amount of metal back-donation in 3c. It is inferred that the TMS complexes 1a-3a, while having higher solubility in scCO2, maintain almost unaltered the electronic and chemical features of their parent compounds. In particular, the role of 1a-3a as catalysts, that is well documented for homogeneous solutions, remains unaltered in the very different scCO(2) environment. The assumption is experimentally validated for 1a by performing with the latter two classic catalytic processes. The first process is the oxidation of PPh3 that is achieved by using molecular oxygen as an oxidant. The second example concerns the epoxidation of cyclohexene achieved in presence of tert-butyl hydroperoxide (TBHP).

Abstract: This article reviews the structural and electronic properties of Fe-2(CO)(9) derivatives with any triad of sigma donor and pi acceptor bridges selected from the series CO, CH2, CF2, SiMe2, GeMe2, InMe and CS. Based on the available structural data and computational results, the bridges other than CO can be catalogued in two different groups. The ligands of group I feature a pivotal carbon atom such as the CO ligand itself (i.e. L = CH2, CF2, and CS). Those of group 11 are characterized by different hetero-atoms at the core of the bridging group (i.e. L = SiMe2, GeMe2 and InMe). The geometry of the inner Fe(mu-L)(3)Fe skeleton changes significantly between the members of the different series. In particular, the range of Fe-Fe distances can be as large as 52.3 pm, the higher and lower limits being found in the system with three InMe bridges and in that with one CO and two CF2 ligands, respectively. It turns out that ligands of group I favor the decrease of the Fe-Fe distance compared to the parent compound Fe-2(CO)(9). On the contrary, those of group 11 favor a larger intermetallic separation. By detailed analyses, the structural effects can be traced back to the sigma donor and pi acceptor capabilities of the different bridges. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: Four heterocycles containing hydroxy and keto functionalities have been tested as chelating agents of beryllium(II). These are in the order (i) 3-hydroxy-2-methyl-4H-pyran-4-one (maltol, Hma), (ii) 5-hydroxy-2-(hydroxymethyl)-4H-pyran-4-one (kojic acid, Hka), (iii) 3-hydroxy- 1,2-dimethyl-4-pyridinone (Hdpp), (iv) 1-(3-hydroxy-2-furanyl)ethanone (isomaltol, Hima). Although the skeletons of the first three species, with one nitrogen or oxygen heteroatom at the six-membered ring, are almost superimposable, straightforward synthesis and crystallization is achieved only for the 1:2 adduct Be(dpp)(2), 1. Also the complex Be(ima)(2), 2, precipitates in high yield but the ima(-) ligand has a different skeletal structure. X-ray determinations of 1 and 2 showed that the Be2+ ion is pseudotetrahedrally coordinated by two chelating ligands with slightly asymmetric Be-O-alkoxo and Be-O-keto bonds. The complex Be(ma)(2) precipitates in low yields together with large amounts of unreacted Hma while, under the same conditions, no trace of the analogous species Be(ka)(2) has been observed. This paper presents the results of potentiometric and NMR studies in the aqueous solutions as well as of DFT structural optimizations for all of the free acids, their associated bases, and the adducts of the type [BeL(H2O)(2)](+)- and BeL2 in the gas phase. It is consistently found that the basicity of the ligands and the stability of their complexes decrease in the order dpp(-) > ma(-) > ka(-) > ima(-). In solution, all of the anionic ligands form adducts of the type [BeL(H2O)(2)](+) at low pH values, whereas higher concentrations of the free anion are required to form 1:2 adducts. The pH, the basicity, and the stability constants of the complexes as well as the formation of competing beryllium hydroxide species are strictly correlated factors for the obtainment of the latter type of adduct. The DFT calculations account nicely for the different donor powers of the various chelates in terms of electronic redistribution and associated energetics.

Abstract: A series of dichloro-bridged arylbicycloheptylpalladium complexes have been synthesized and characterized by means of NMR spectroscopy. The compound [(C16H19)PdCl](2).CH2Cl2 with ortho and para methyl substituents at the arene has been characterized by means of X-ray diffraction techniques. The C-ipso atom of the arene lies almost at the fourth planar coordination site of the metal [Pd-C-ipso = 2.22(1) A (average)], and due to the arene's tilting, the substituted C-ortho atom is relatively close to the metal atom [2.54(1) Angstrom (average)]. The coordinated C-ipso-C-ortho linkage, in a seemingly dihapto coordination, is anti with respect to the CH2 bridge of the bicycloheptyl unit. Variable-temperature NMR experiments for the para-substituted dinner 9 reveal restricted rotation of the two aryl groups about the corresponding C-C,p., bonds (DeltaE less than or equal to 17 kcal mol(-1)). DFT-B3LYP calculations have been carried out on the known and similar monomer (phenylbicycloheptenyl)Pd(PPh3)l (4) and its related substituted derivatives, The essential results are as follows: (i) The potential energy surface for twisting the phenyl ring away from the symmetric eta(1) coordination in 4 is very flat (DeltaE less than or equal to 1 kcal mol(-1)) whereas an Atoms in Molecules analysis excludes the existence of an actual Pd-C-ortho bond in the seemingly eta(2)-type conformer, (ii) Complete rotation of the unsubstituted phenyl ring is not facile but feasible, A significant strain affects the transition-state structure featuring a Pd-HCaryl agostic-type bond. The calculated destabilization of 10.3 kcal mol(-1), with respect to the ground state, can be compared to the experimental barrier of the dimer 9. (iii) Various methyl-substituted derivatives of 4 have been optimized, and their structural and energetic trends are discussed. An almost ideal eta(1) coordination is shown by the anti conformer of the C-otho-substituted complex due steric effects. For all of the other cases, a slipped eta(2) coordination may be described, As a general conclusion, the unsaturated metal center receives pi electron density of the arene mainly through its C-ipso atom. The effect may be slightly improved if the C-ortho atom also gets closer to the metal, but in no case, does the slipped eta(2) coordination seem to be crucial for the stability of the system.

Abstract: Reported complexes of general formula L4M(phenylenediamido-o), where M is a metal of group 6 in various oxidation states (VI, IV, H and 0), are analyzed. The possible structural deviations from the most symmetric pseudo-octahedral structure (the one with the, chelate in the equatorial plane) are evaluated, case by case, in view of their electronic origin. The analysis is based on systematic DFT calculations and their qualitative MO interpretation based on perturbation theory concepts.

Abstract: The reaction between the linear trinuclear complex [Pt{Fe(CO)(3)(NO)}(2)(PhCN)(2)] and Ph-2(2-C5H4N)PSe led to the isolation and characterization of the 46-electron cluster [(CO)(3)Fe(mu(3)-Se){Pt(CO)P(2-C5H4N)Ph-2}(2)] (1), whose structure has been determined by X-ray diffraction methods. The cluster typology, which consists of an open triangle Pt-Fe-Pt capped by a mu(3)-Se atom, is rather rare. The chemical bonding in I and in similar systems has been analyzed through density functional theory (DFT) and qualitative MO approaches. A strict analogy with the well understood L2M(mu acetylene) ML2 systems is invoked by considering I as formed by the (CO)(3)FedropSe tetrahedral unit stabilized by sidewise interactions of the triple bond with two d(10)-L2M fragments. Otherwise, the 18-electron (CO)(3)FedropSe monomer is unstable as an isolate molecule. This is confirmed by our DFT calculations that indicate how the well characterized dimer (CO)(3)Fe(mu-Se-2)Fe(CO)(3) lies as much as, approximately, 58 kcal mol(-1) deeper in energy. Finally, by considering an analogy with [L2M(mu-dichalcogen)ML2](0,+2) redox systems (M = Pd, Pt), reduction of 1 to a dianion has been hypothesized and the structure of the latter has been tentatively explored by DFT calculations. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: Reaction of [(triphos)Re(CO)(2)(OTf)] (1) [triphos = MeC(CH2PPh2)3; OTf = OSO2CF3] with P4S3 and P4Se3 yields pairs of coordination isomers, namely, [(triphos)Re(CO)(2){eta(1)-P-apical-P4X3}](+) (X = S, 2; Se, 5) and [(triphos)Re(CO)(2){eta(1)-P-basai-P4X3}](1) (X = S, 3; Se, 6). The latter represent the first examples of the eta(1)-P-basal, coordination achieved by the P4X3 molecular cage. Further reaction of 2/3 and 5/6 mixtures with I affords the dinuclear species [{(triphos)Re(CO)(2)}(2){mu,eta(1:1)-PapicalPbasalP4X3}](2+) (X = S, 4; Se, 7) in which the unprecedented M-eta(1)-P-basal/eta(1)- bridging coordination of the P4X3 Molecule is accomplished. A theoretical analysis of the bonding properties of the two coordination isomers is also presented. The directionality of apical vs basal phosphorus lone pairs is also discussed in terms of MO arguments.

Abstract: The article reports the first transition metal complex of methanesulfonylhydrazine, a molecule known for important pharmacological and chemical applications. The complex [ZnCl2(MSH)(2)], MSH = CH3SO2NHNH2, has been synthesized and its structure characterized by means of Xray analysis. The coordination geometry is pseudo-tetrahedral and the ligand MSH adopts a conformation that was found by previous ab initio calculations to be a transition state and not a stationary point for the free ligand.

Abstract: A novel synthetic route to [Ni(oda)(H2O) 3].1.5H(2)O 1 and [Co(oda)(H2O)(2)].H2O 2 [oda = oxydiacetate, O(CH2CO2-)(2)] is presented. These complexes react with bidentate N-donor ligands to yield compounds of general formula [M(oda) ( N-N)(H2O)] (M = Ni, N-N = bipy, 3; phen, 5; tmeda, 7. M = Co, N-N = bipy, 4; phen, 6; tmeda, 8), which can be isolated as crystalline solids with different numbers of hydration water molecules. The molecular structures of the compounds 3 and 4 have been shown by X-ray analysis to crystallize with 2.5H(2)O. In these compounds, the oda ligand adopts a planar, tridentate conformation (mer) in binding the octahedral metal centre. By contrast, the recent X-ray characterization of 1 has shown that the oxydiacetate ligand completes the octahedral coordination of the metal in the fac arrangement. The DFT method has been adopted to investigate theoretically the flexibility of this ligand. First, calculations have been carried out on the free oxydiacetic acid (H(2)oda) and its anion. Then, a model of 1 has been studied in order to find the geometric and energetic implications for the alternative oda conformations (mer and fac) in reaching the octahedral environment about the metal. In general, the geometric parameters are in good agreement with those available from crystal structures. From the energetic viewpoint, the mer conformation is favoured by a small energy difference (2.0 kcal mol(-1)). On the other hand, the fac conformation in 1 may be justified with the presence of the water hydration molecules (not considered by our model) which may deeply affect the energetics of the overall system by forming strong hydrogen bonds with the oxydiacetate ligand.

Abstract: Polynuclear iron carbonyl complexes have been prepared only recently, in, which bridging carbonyl ligands were replaced by isolobal monoalkylindium groups. A theoretical analysis of the compounds (CO)(3)Fe-(mu -CO)(3-x)(mu -InCH3)(x)Fe(CO)(3) (x = 0, 1, 2, 3) shows the specific influence of a stepwise substitution of CO by Rln-(I) fragments on their electronic, structural, and bonding properties. This influence is determined by the order of the a donor [Rln(I) much greater than CO(bridge)] and pi acceptor [RIn-(I) < CO(bridge) <much less than> CO(terminal)] capabilities of these ligands.

Abstract: BFT/B3LYP calculations on known and hypothetical doubly bridged Cu(I)-Cu(I) dimers and other d(10)-d(10) analogues have been carried out. The bridging ligands may he only a donors (hydrides) or have added pi -donor (halides) or pi -acceptor (carbonyls, as get unknown) capabilities. In particular, the few reported LnCu(mu -C drop CR)(2)CuLm frameworks have been investigated, The latter are symmetric (type b) or asymmetric (types a and b), depending an the nature and number of terminal ligands (n = 1, 2; m = 1, 2). Beside the accurate geometric and energetic computations, the nature of the chemical bonding is explored in terms of perturbation theory arguments (EHMO approach). Thanks to the cs donor power of the bridges, electron density is driven into the bonding combinations (sigma and pi) of empty metal a and p(pi) orbitals, In the presence of pi -donor ligands, population of the corresponding sigma* and pi* levels occurs and the M-M bond vanishes. In contrast, insufficient hack-donation from copper d orbitals prevents the formation of bridged carbonyl dimers and trigonal-planar monomers are favored, A case study is that of the heterobinuclear d(10)-d(10) complex (CO)(2)-Cu(mu -CO)(2)Co(CO)(2) where the lone pairs of the CO bridges are preferentially directed toward cobalt for electronegativity reasons. X similar situation is highlighted for the model (PH3)(2)-Cu(mu -C drop CH)(2)Cu(PH3) (type b), where both bridges orient toward the unique fragment (PR3)Cu because of the different hybridization of L2M and LM sigma orbitals. In the species LnCu(mu -C drop CH)(2)CuLn (n = 2 or n = 1, type a or c), the potential energy surface for the symmetric to asymmetric rearrangement, of the central Cu2C2 ring is quite flat. However, a symmetric Cu-2(mu -C drop CR)(2) framework is achieved with eta (2)-bound alkynes (type c), This is attributable to the pi* levels of the latter Ligands, which stabilize the metal p(pi) orbitals involved in bridge bonding, The asymmetric Cu2C2 arrangement is preferred again in models where the terminal alkynes are substituted for by single phosphine ligands.

Abstract: The reaction of Na-2[Fe(CO)(4)] with Br2CF2 in n-pentane generates a mixture of the compounds (CO)(3)Fe(mu -CO)(3-n)(mu -CF2)(n)Fe(CO)(3) (2, n = 2; 3, n = 1) in low yields with 3 as the main product. 3 is obtained free from 2 by reacting Br2CF2 with Na-2[Fe-2(CO)(8)]. The non-isolable monomeric complex (CO)(4)Fe=CF2 (1) can probably considered as the precursor for 2. 3 reacts with PPh3 with replacement of two CO ligands to form Fe-2(CO)(6)(mu -CF2)(PPh3)(2) (4). The complexes 2-4 were characterized by single crystal X-ray diffraction. While the structure of 2 is strictly similar to that of Fe-2(CO)(9), the structure of 3 can better be described as a resulting from superposition of the two enantiomers 3a and 3b with two semibridging CO groups. Quantum chemical DFT calculations for the series (CO)(3)Fe(mu -CO)(3-n)(mu -CF2)(n)Fe(CO)(3) (n = 0, 1, 2, 3) as well as for the corresponding (mu -CH2) derivatives indicate that the progressively larger sigma donor and pi acceptor properties for the bridging ligands, in the order CO < CF2 < CH2, favor a stronger Fe-Fe bond.

Abstract: The oxo-vanadium(IV) complex VO(oda) (H2O)(2) (I) (oda = oxydiacetate, O(CH2COO-)(2)) was obtained by reaction of aerobic aqueous solutions of VO(acac)(2) with oxydiacetic acid, O(CH2COOH)(2). The antiapoptotic biological activity of I was studied in insulin-producing cells. Chemically generated nitric oxide (NO) triggers apoptotic events, such as the appearance of oligonucleosomes in cytosol, and this response was prevented by the presence of 1 in the culture medium. The molecular structure of 1 has been determined by X-ny diffraction analysis. (C) 2000 Elsevier Science S.A. All rights reserved.

Abstract: The X-ray structure of 1-(N-ethyl-1-sulphonate-4-pyridinio)-2-[N-methylpyrrol-2-yl]ethene . H2O is reported. The molecule belongs to the spatial group P2(1)/n with unit cell constants a=6.841(5), b=21.259(5), c=10.195(5), beta=110.262(15)degrees. The compound was characterized by means of UV-vis and steady-state fluorescence spectroscopies. In the visible region the powder shows a blue shift of the main absorption band with respect to the solution. Analogously a marked red shift and decrease of intensity is observed in the fluorescence spectra. Packing effects are most likely at the origin of the difference. The molecule is expected to have promising proprieties for two-photon pumped (TPP) frequency-upconversion using experiments.

Abstract: Folding of ene-diamido chelates in d(0) metal complexes is generally attributed to a saturating pi interaction of the metal with the central C=C linkage. A MO analysis of selected models (DFT and EHMO levels) shows that the latter interaction is negligible (negative M-C overlap populations). Moreover, a detailed analysis of the orbital interactions highlights a two-electron how from filled p, orbitals of the nitrogen atoms toward the metal consequent to ligand folding. Conversely, it is evident that the C=C pi-electron cloud is only barely affected by the contacts with the metal atom.

Abstract: The compound {Ru-3(CO)(9)[mu-P(NPr2i)(2)](3)}{Ru-6(CO)(15)(mu(6)-C)[mu-P(NPr2i)(2)]} (1), Obtained via the addition of PCl-(NPr2i)(2) to K-2[Ru-4(CO)(13)], crystallizes in the monoclinic space group P2(1)/c with a = 15.537(8) Angstrom, b = 36.151(16) Angstrom, c = 19.407(5) Angstrom, beta = 91.14(2)degrees, Z = 4, and R = 0.069 for 8006 observed reflections. The unit cell is unusual in that it contains both a typical octahedral RU6 cluster anion (la), featuring an encapsulated carbide, and a symmetrical phosphido bridge, in addition to a 50-electron trinuclear cluster cation {Ru-3(CO)(9)[mu-P(NPr2i)(2)](3)}(+) (Ic). The latter, with approximate D-3h symmetry, exhibits long Ru-Ru distances (greater than or equal to 3.15 Angstrom). Among the family of clusters with M-3(mu-PR2)(3) cores and different numbers of both electrons (TEC) and terminal ligands (LxLyLz), Ic is unique in that it is a 333 stereotype with 50 valence electrons. MO calculations permit us to predict the existence of redox congeners of Ic clusters and related 48e Re-3 clusters. This work also presents a summary of the relationships between the electronic and the geometric structures for all known M3LxLyLz(mu-PR2)(3) species. The basic stereochemical features are influenced by the total-electron count and, hence, by the degree of M-M bonding, as well as the remarkable flexibility of the phosphido bridging ligands. The mu-PR2 ligands need not necessarily lie in the M-3 plane, and a wide range of M-P-M angles (as small as 72 degrees or as large as 133 degrees) have been observed.

Abstract: A different number of bridging carbonyls is found in bi- or trinuclear clusters having the title formulas. Comparative calculations at the SCF, MP2, and DFT levels of theory show that only the latter is able to describe properly the energetics of various isomers of the whole triad. For the first-row transition metal, DFT gives excellent agreement with the experimental structures, whereas the MP2 approach fails completely. Conversely for the second- and third-row metals, the best agreement with the experiment is obtained by the MP2 optimizations. The quantitative computational results, associated with a qualitative MO analysis, allow one to conclude that the structural preferences are determined by a critical balance of metal-bridge bonding, metal-metal bonding, and intermetallic repulsion. Although the M-M bond order is expected to be 1 in all cases, the bridge-supported bond is experimentally and computationally shorter than the unsupported one. By contrast, the trend for the overlap population (OP) is reversed, with even negative Values for the shorter bridge bonds. For the latter, only a weak attractive interaction stems from the almost pure t(2g) orbitals, taken as metal lone pairs or eventually responsible for back-donation (formation of metal-bridge sigma bonds). Thus, the negative OP values are consistent with a prevailing repulsion between the latter levels. In the iron systems, with more contracted metal orbitals, the direct metal-metal repulsion is relatively weak while the metal-bridge bonds are sufficiently strong. This is not equally true for the more diffuse ruthenium and osmium orbitals, so the alternative nonbridged structure is preferred.

Abstract: The tetrakis(1-methylcytosine) complex [Pt(1-MeC-N-3)(4)](NO3)(2) (1) in its head-tail-head-tail (htht) rotamer form binds upon deprotonation of the exocyclic amino group of 1-MeC, two Cu-II at either side of the central Pt-n to give trinuclear cations [Pt(1-MeC-)(4)Cu2X2](n+) (2) with X = Cl-, H2O, NSC-, N-3(-). X-ray crystal structure analyses of [Pt(1-MeC-)(4)Cu-2(Cl)(1.6)(H2O)(0.4-). 1.6H(2)O (2a) and [Pt(1-MeC-)(4)Cu2-(H2O)(2)](NO3)(2). 3H(2)O (2b) reveal short Cu-Pt distances of 2.519(2)- 2.531(2) Angstrom. These compounds are diamagnetic as a result of the strong exchange interaction (J = 980 +/- 30 cm(-1) (2a)), despite a separation of approximately 5 Angstrom between the Cu-II centers. MO considerations are in agreement with the assumption that Pt-II acts as a mediator between the two Cu-II centers. A second trinuclear complex derived from the hhhh rotamer of 1 containing both Co-III and Na+ [Pt(1-MeC-)(4) Co(H2O)Na](NO3)(2). 2H(2)O (3) has been isolated in very low yield and characterized by X-ray structure analysis. The Com center is bound to four amide N atoms and forms a bond to Pt (2.402(2) Angstrom), whereas the Na+ ion is bound to the four oxygen atoms of the nucleobase, A general route for the preparation of heteronuclear MPtM and MPtM' compounds is described.

Abstract: A comprehensive study of the molecule of methanesulfonic acid hydrazide (MSH) is presented. The X-ray structure shows that in the crystal two centrosymmetrically oriented MSH molecules are Yield together by N-H ... N hydrogen bonding interactions. This feature is unprecedented for the known arylsulfonic hydrazide analogues. The energetics of the various MSH staggered conformers and the stabilisation due to dimerisation are evaluated by HF ab initio calculations. Moreover, the pathways that interconvert the conformers as well as their enantiomers are outlined. The transition states between conformers correspond to the eclipsed conformation about the S-N linkage while those between enantiomers require planarisation of the N atom bound to sulfur. Some interconversions require two steps and two barriers to be bypassed. The IR and Raman spectra of MSH have been recorded and a normal coordinate analysis (NCA) has been carried out. The assignments have been double-checked through the ab initio calculated frequencies. The latter techniques also allow evaluation of the normal modes of vibration due to interacting MSH monomers, which can be experimentally detected.

Abstract: The reactions of I-2 with ptc and ttb (title ligands) have been investigated in CHCl3 solution at different temperatures by spectrophotometry. A least-squares method procedure provided evidence for the formation of the 1:1 adducts. Crystals of the latter have been analyzed by X-ray diffraction methods (both monoclinic, P2(1)/c; ptc.I-2, a = 8.691(6) Angstrom, b = 9.010(6) Angstrom, c = 13.237(5) Angstrom, beta = 103.43(2)degrees, Z = 4, R = 0.0305; ttb.I-2, a = 12.090(6) Angstrom, b = 6.433(5) Angstrom, c = 15.731(6) Angstrom, beta = 99.30(2)degrees, Z = 4, R = 0.0419). Both structures show that the thionic sulfur tin any case a CS3 group inserted in a ring) is bound almost collinearly with the diiodine molecule. The IZ(S-I) separations are 2.755(2) and 2.805(3) Angstrom in the ptc.I-2 and ttb.I-2 adducts, respectively, while d(I-I) is practically the same (2.812(2) Angstrom). An evident stereochemical difference is that the S-I-I moiety is nearly coplanar with the CS3 group in ptc.I-2 while it is upright in ttb.I-2; However, the feature is not expected to cause a major electronic difference. In order to reproduce the structural features, different ab initio approaches have been attempted, with the best results being obtained with the density functional method (DPT). Despite the S-l distances which are slightly longer than the experimental ones (by ca. 0.25 Angstrom), the distribution of filled and empty frontier molecular orbitals (MOs) allows a good interpretation of the visible spectra. Also a rationalization of the sigma electronic density distributed over the three centers S-I-I has been attempted by qualitative MO theory (EHMO method). Provided the good agreement with the higher level calculations, the perturbation theory arguments highlight the variable sp hybridization at the central iodine atom as the electronic factor of importance. The strength of the donor (D) affects significantly the redistribution of six electrons over four atomic orbitals, and the classic model is revised as a four-orbital/six-electron one. Thus, it is pointed out that a major four-electron repulsion is exerted over the D-I or the I-I linkages with major consequences for their respective lengths.

Abstract: A review of all of the known structural stereotypes of dimeric Pt(I) or Pd(I) systems with at least one bridging phosphido group is presented. The nature of the direct metal-metal interaction is affected by the number, nature, and disposition of the various coligands. It appears that in some cases a bent M-M bond is by itself a center of nucleophilicity. On the other hand, it is experimentally known that the bridging phosphido ligand is a competing nucleophile, giving rise to an agostic interaction in Pd-2, but not in Pt-2, derivatives. MO theory is used to outline the major electronic features and correlation between different prototypes. Besides a qualitative approach based on the EHMO method, DFT and MP2 methods were used to reproduce the experimental structures and to explore the possibility of unknown tautomers.

Abstract: Electrochemistry of M(eta(2)-C-60)(CO)(2)(phen)(dbm) (M = W 1, Mo 2; phen = 1,10-phenanthroline; dbm = dibutyl maleate) shows that the complexes undergo four sequential reduction processes. As with foe C-60, the first three electrons add reversibly (even if the relevant potentials are shifted ca. 0.15 V toward negative values), whereas the fourth reduction features chemical irreversibility. Cyclic voltammetry gives evidence that, as a consequence of the latter process, the metal fragment decomplexes and [C-60](3-) is released. In good agreement with this picture, a qualitative MO approach shows four close LUMOs for the ground state structure of the uncharged complexes. The first three levels are delocalized over C-60 (somewhat extended to the dmb pi system), while the fourth one is metal-fullerene antibonding (back donation d(n) --> pi* C-60) and its occupation causes fulleride dissociation. The EPR spectra of the electrogenerated [1](-) and [2](-) monoanions are significantly different from that of [C-60](-) and seem suggestive of metal character for these radical species. At present, this result is unexpected in that the unpaired electron in the anions [1](-) and [2](-) should be intuitively centered on the coordinated fullerene.

Abstract: The transformation of acetylene into vinylidene, as promoted by the metal fragment [(pp(3))Co](+) [pp(3) = P(CH2CH2PPh2)(3)], is unimolecular and features the hydride-acetylide species as an intermediate. The paper describes a detailed ab initio study of the reaction, in particular with regard to the step involving 1,3-H shift. The best computational results are obtained by mimicking the pp(3) Ligand with actual ethylenic chains rather than with single PH3 molecules. The keypoints along the two-step reaction path (pi-acetylene, hydride-acetylide, and vinylidene complexes, as well. as intermediate transition states) have been optimized for Co-I and Rh-I derivatives at the MP2 level. For the fragment [(pp(3))Co](+), the barrier associated with transformation of the hydride-acetylide intermediate to vinylidene (20.6 kcal/mol) is easier to surmount compared to that for reversion to the reactants (28.6 kcal/mol). The situation is reversed for the analogous Rh-I system, with the initial pi-acetylene adduct being slightly more stable. Although higher in energy, the hydride-acetylide species is the experimentally detected product of the reaction of acetylene with the fragment [(pp(3))Rh](+). The salient chemical aspects of the 1,3-H shift are discussed in terms of perturbation theory arguments. Parallel EHMO calculations, which have provided a relatively good consistency with the ab initio results, allow the proposal of an orbital rationale for the mode of migration of the hydride ligand along the substantially Linear Co-C-alpha-C-beta grouping.

Abstract: The structure of the t-butyldiazopyruvate (1), an intermediate in the synthesis of DPTA-like ligands (DTPA = diethylenetriaminepentaacetic acid), has been determined by X-ray analysis. The molecule, which lies in a crystallographic mirror plane, is characterised by a cis arrangement of the two carbonylic oxygen atoms. Molecular mechanics and quantum mechanical molecular orbital calculations, at EHMO and ab initio levels, were performed in order to investigate the conformational properties of the isolated molecule. The packing effect has been examined by means of the program PROMET (5) by calculating the packing energies for a number of possible crystal structures. It is found that the energetic differences calculated for the various effects are small and comparable. Thus, the cis conformation found in the experimental structure is not in deep contrast with the theoretical calculations for the free molecule which favor the trans conformer. An energy barrier is detected by all of the computational methods when two CO functional groups lie in orthogonal planes. Importantly, a search in the Cambridge Structural Database indicates that all of the torsional conformations are possible in the 0-180 degrees range. The complexity of the contrasting energetic factors is presented in some detail.

Abstract: Eleven antimony compounds RSb[(CH2)(3)](2) NR' and eight germanium compounds R2Ge[(CH2)(3)](2) NR' (some combinations of R = Cl, I, NCS, OSiPh3, Ph, and NR' = NMe, NBz, NBui) have been synthesized by diGrignard reactions and subsequent treatment with AgSCN, Ph3SiONa or PhLi. The compounds were compared to 14 Sn, 1 As, 5 Sb and 4 Bi analogues taken from the literature. Evidence is provided for 1,5-chelation Sb ... N via crystal structure determinations (5 + 1 (lit) Sb compounds), C-13- and Si-29-NMR chemical shifts;Sb-121 and I-127 Mossbauer data (11 and 3 Sb compounds, respectively), cyclic voltammetry (9 Sb compounds) and semi-empirical MO calculations on the Extended Huckel level (8 present compounds and 4 models). The ligand sequence (Cl < I < NCS) holds for an increasing approach N ... Sb. The effect of electronegativity and the effect of n- or pi --> sigma* charge transfer into the LUMO at Sb are counteractive. The intramolecular donor strength increases in the following order: NBu' < NBz < NMe. Simultaneously a decrease of the absolute value of the Sb-121 quadrupole coupling constant from Mossbauer spectroscopy occurs. The postulation of a control of the Sb ... N chelation by frontier orbital interactions is supported by the CV results in CH2Cl2 at a Au/Hg electrode. It is proved by scan rate dependence that the observed reversible reduction/oxidation process is diffusion controlled, and by coulometry that the reduction takes place by transfer of one electron. (C) 1998 Elsevier Science Ltd. All rights reserved.

Abstract: A new family of organometallic Ni(II) benzonitrile derivatives bearing strong donor substituents of general formula [Ni(eta(5)-C5H5){P(C6H5)(3)}NCC6H4R][PF6] (R = NH2, N(CH3)(2), C6H5, OCH3, H, F) has been synthesized. A structural study by X-ray diffraction of the compound with R = NH, showed crystallization on the centrosymmetric monoclinic space group, P2(1)/n, with a quasi planar structure of the coordinated nitrile. First and second hyperpolarizabilities evaluated by hyper-Rayleigh scattering and Maker fringe techniques showed essentially the same values for the complexes and the uncoordinated chromophores. Theoretical studies by the extended Huckel method found a small amount of pi delocalization due to the cancelling effects on the interaction of the metal fragment orbitals with pi and pi* nitrile orbitals. (C) 1998 Elsevier Science S.A.

Abstract: New consideration of the chemical bonding in [L3M(mu-P-3) M'L-3](n+) complexes (triple-deckers with bifacial coordination of the unit P-3) focuses on the active role played by the P-3, sigma bonding network. The argument, developed from an extended Huckel molecular orbital (EHMO) analysis, is based largely on the rigorous symmetry properties of the MO interactions. In particular, for 30 electron species, only the in-plane sigma bonding orbitals of P-3, can match the in-phase metal d(pi), combinations (e' symmetry for an ideal D-3h model). Additionally, an in-phase combination of metal sigma-hybrids interacts with the total-symmetric sigma bonding MO of P-3, (a(1)'). Only through the latter interactions, are the low lying and empty metal levels partially saturated. On increasing the total electron count, the donor-acceptor interactions of e' symmetry are reduced and, for 34 electron systems, the attractions transform into electron repulsions without destroying the primary triple-decker assembly. Finally, the role of the P-3, sigma orbitals is re-examined for some monomeric complexes of the type LnM(eta(3)-P-3) and for the white phosphorus molecule (P-4) itself. Thus, the variations of the P-P bond lengths can be interpreted in terms of a different involvement of the P-3 a orbitals for both the mononuclear and binuclear adducts. (C) 1998 Elsevier Science S.A. All rights reserved.

Abstract: The reaction of trans-Mo(C2H4)(2)(PMe3)(4) with PhC equivalent to CCOOEt affords (PhC equivalent to CCOOEt)(3)(PMe3) (1). The molecular structure of 1 has been determined by means of X-ray analysis. NMR studies have shown that 1 undergoes a dynamic process in solution due to the alkyne rotation. The related energy barrier is experimentally determined to be 15.1 kcal mol(-1). The well established EHMO theoretical arguments, which uniquely explain the chemical bonding in these complexes, are extended to interpret the nature of the barrier and its dependence on the pi-donor or pi-acceptor capability of the axial ligand. The conceptual continuity between the chemical bonding of d(6) metal species coordinated by at least two alkyne molecules (e.g. M(alkyne)(2)L-4, M(alkyne)(2)L-2 and M(alkyne)(3)L) is highlighted. (C) 1998 Elsevier Science S.A. All rights reserved.

Abstract: The electronic structure of the trinuclear compound [(Ph3P)(OC)(2)Ru{mu-C6H4(NH)(2)}Mo(CO)(2){mu-C6H4(NH)(2)}- Ru(CO)(2)(PPh3)], with a diamidolene ligand riding upright over each of the quasi-collinear Ru-Mo linkages, has been investigated by means of the extended-Huckel method and a graphic interface. The ability of the dianionic riders to donate eight electrons to adjacent metals is interpreted, similarly for related Ru, dimers, in terms of perturbation theory. The molecule's distortion, from the highest possible C-2v to C-2 symmetry was parametrized and the effects on the overall chemical bonding evaluated. The nature of the extended Ru-Mo-Ru linkage is addressed in some detail. The electronic redistribution over the ruthenium and molybdenum atoms is discussed in terms of their limiting oxidation states. Inferences are made as to the structure of a chromium analogue of the MoRu2 compound, as yet unknown.

Abstract: The interconversion between the valence tautomers 1,6-methano[10]-annulene (1) and dinorcaradiene (2) was computationally investigated by qualitative extended Huckel molecular orbital methods (with CACAO visualization) and quantitative semiempirical and ab initio methods. The fundamental bonding interactions are described in terms of perturbation theory arguments for both tautomers, the influence of pi-acceptor or pi-donor substituents at the C11 position is rationalized, and bonding changes during the interconversion are monitored. The electronic molecular structural preferences can be modified by through-space interactions, and residual C1-C6 a bonding remains even at the >2.1 Angstrom separations of the annulenic structures. Hitherto unprecedented calculations of the ge ometries of C11 homo-disubstituted derivatives of 1 and 2 have been carried out at semiempirical and ab initio levels to identify the mon stable tautomer and find whether two stable minima may exist for each derivative. Searches at the AM 1 level to determine transition-state structures for the interconversion of 1 to 2 and of their C11 homo-disubstituted derivatives ale also reported.

Abstract: The reaction of [(dppe)(CO)(2)(MeCN)W=CCH=CCH2(CH2)(n)CH2CH2][BF4] (dppe = 1,2-bis(diphenylphosphino)ethane; n = 1 (la), 4 (Ib)) with S2CPCy3 in methanol for 10 min affords [(dppe)(CO)(2){kappa(S)-S2CPCy3}W=CCH=CCH2(CH2)(n)CH2CH2][BF4] (n = 1(2a), 4 (2b)). Similarly, the complexes [(dppe)(CO)(2){kappa(S)-S2P(OEt)(2)}W=CCH=CCH2(CH2)(n)CH2CH2] (n = 1(3a), 4 (3b)) have been prepared by the reaction with [NBu4][S2P(OEt)(2)]. When la and Ib are eacted with S2CPCy3 in methanol for 24 h, the complexes [(dppe){kappa(3)(S,C,S)-S2CPCy3} W=CCH=CCH2(CH2)(n)CH2CH2][BF4] (n = 1 (4a), 4 (4b)) are formed. Moreover, when la and Ib are reacted with NaS2CNR2 (n = Me, Et), the ketenyl complexes [(dppe)(CO){kappa(2)(S,S)S2CNR2)W{kappa(2)(C, C)=C-(C=O)CH=CCH2(CH2)(n)CH2CH2}] are formed (R = Me, n = 1 (5a), 4 (5b); R = Et, n = 1 (5c), 4 (5d)) subsequent to the coupling between the carbyne group and carbon monoxide. IR and H-1, P-31, and C-13 NMR data are reported for all of the species. The molecular structure of compound 4a, determined by X-ray diffraction methods, shows that the S2CPCy3 ligand is bound to the metal in a pseudoallylic kappa(3)(S,C,S) fashion. The metal coordination is completed by the W=C triple bond of the alkenyl-carbyne group and by two additional W-P bonds. Since the metal center in 4a is formally a deficient 16-electron species and since the synthetic route has clearly indicated that an additional CO ligand is preferentially rejected by the metal, a reasonable interpretation of the bonding has been sought by means of EHMO calculations. Not only has the coexistence between the dithiocarboxylate and the carbyne ligands been elucidated, but also the strength of the W-S bonds is accounted for in comparison with the weaker W-S bonds observed in other saturated W complexes containing a kappa(3)(S,C,S) dithio ligand. Finally, qualitative theoretical considerations are presented for the different behavior of the dithiocarbamate ligand.

Abstract: This paper analyzes from the theoretical viewpoint (ehmo calculations with the graphic interface of CACAO) the bonding capabilities of conjugated pi-ligands containing the unit XCCX (X = S, NR, PR) when they ride on binuclear fragments of the type L6M2 (sawhorse). The stereochemistries are monitored for a number of structures retrieved from the Cambridge Database. In the starting prototype, e.g. the complex [(CO)(2)(PPh3)Ru{mu-1,2-(NH)(2)C6H4}Ru(PPh3)(CO)(2)] with 34 valence electrons, the rider donates 4 electron pairs to two d(7) metals and a single M-M bond is featured. In spite of the predicted electron similarities between diamidolenes and dioxolenes, no case has been reported of a catecholate ligand adapting to the same riding mode and therefore being able to donate a total of 4 lone pairs to the metals, The different behavior is largely ascribed to poor interactions of b(1) symmetry when the donor atoms of the rider are of higher electronegativity. Stronger donors, such as phosphorus and sulfur, still support the riding mode but elongate understandably the M-M bond. In presence of d(6) metal atoms, the riding ligand bends over one of the metals and also donates the electrons of the C=C pi-bond itself. Alternatively, an extra ligand (terminal or bridging) can be added to the primary framework M2L6. In all of the latter cases, the various factors affecting the nature of the M-M bond are theoretically interpreted.

Abstract: The reaction of Fe(BDA)(CO)(3) (BDA = benzylidene acetone) (1) with the dithiocarboxy adduct S(2)CPR(3) affords complexes of formula Fe(eta(3)-S(2)CPR(3))(CO)(3) (2). The pseudoallylic coordination of the SCS grouping is ascertained by an X-ray determination on the cyclohexyl derivative, 2a. The primary geometry of the complexes 2 is also maintained upon the electrophilic attack of a methyl cation to one of the sulfur atoms. This piece of information was obtained by solving the structure of the complex [Fe{eta(3)-CH3SC(S)PCy(3)}(CO)(3)]CF3SO3 (3a). Unfortunately, the complete set of X-ray data for the latter cannot be completed due to the high disorder affecting the CF3SO3 anion. Altogether, these and the other experimental data available for a variety of ML(n)(S(2)CX) complexes have prompted a general theoretical study (based on EHMO calculations) of the factors affecting the eta(3)-eta(2) coordination dichotomy of the dithiocarboxy ligands, S(2)CX. The Fe(CO)(3) fragment, also in view of one extra vacant coordination site, appears better suited than other 14-electron metal fragments, such as Mo(CO)(2)(Pr-3)(2) and Fe(PR(3))Cp, to favor the pseudo-allylic coordination of the SCS grouping. In any case, an energetic barrier is observed for the eta(2)/eta(3) interconversion. This is due to an initial four-electron repulsion between the overall pi-bonding S(2)CX level and a d pi(perpendicular to) metal orbital. A HOMO-LUMO avoided crossing transforms the repulsion into a bonding attraction between the metal and the central carbon atom.

Abstract:

Abstract: The fluxional behavior observed for the M(eta(3)-S-2-(CPMe(3))(CO)(2)(PMe(3))(2) (M = Mo, W) complexes is analyzed using a frontier orbital approach and extended Huckel calculations.

Abstract: The complexes [{(triphos)M}(2)(mu-eta(3),eta(4)-SC4H4)](Y)(2) (M = Ir, Y = BPh(4); M = Rh, Y = PF6) are obtained by reaction of thiophene with either [(triphos)Ir(eta(4)-C6H6)]BPh(4) in Me(2)SO at 80 degrees C or [(triphos)RhCl(C2H4)] + TlPF6 in THF at room temperature; a single-crystal X-ray diffraction analysis of the iridium complex shows that an open C4H4S moiety sits astride two metal centres; the bridge-bonding mode implies the sharing of four electron pairs with the metals.

Abstract: The hetero-bimetallic Ind complexes (Ind = indenyl anion) with the formula (CO)(3)Cr(mu-Ind)RhL(2) (L = CO, ethylene; L(2)= COD, COT, NBD) have two possible conformers, one with the metals at antipodal sides of the condensed bicyclic polyene (antarafacial, anti) and the other with both metals on the same side (cofacial, syn). These systems can be considered to contain 34 valence electrons by assuming that Ind is capable of donating all of its ten rr electrons to the metals. A qualitative MO analysis, based on EHMO calculations, is presented. The anti conformer is compared to the homometallic 34e species, namely [CpRu(mu-Ind)RuCp](+). Here, the pseudo-symmetrical relationship between the two identical metal fragments strongly suggests that one Ind sigma-bonding MO donates part of its electron density to each of the two metals, which thus become formally saturated (36e). This argument is extended to the anti Cr-Rh derivative in which a critical role is played by an empty FMO of the (CO),Rh(I) fragment with pi(11), symmetry (i.e. lying in the molecular mirror plane). The latter, in spite of the large pi co* contributions, has good acceptor capabilities at the metal. The calculations suggest that the so-called indenyl effect, namely the increased rate of substitution of one CO ligand by another sigma donor (e.g. a phosphine), although a structurally dynamic process, also depends on the electron density which accumulates on the aforementioned pi(11) FMO. In the series (eta(5)-C5H5)Rh(CO)(2), (eta(5)-C7H9)Rh(CO)(2) and eta(5)-[(CO)(3)CrC7H9)]-Rh(CO)(2) the progressively reduced donor capabilities of the cyclic polyene toward the (CO)(2)Rh(I) fragment induce a larger electrophilicity in rhodium, in good agreement with the experimental data on substitution reactivity. Finally, the electronic features of the cofacial conformer (CO)(3)Cr(mu-Ind)Rh(CO)(2) are examined. In spite of the long Cr-Rh separation ( > 3 Angstrom) the graphically illustrated MO analysis indicates a direct, heterodox, intermetallic linkage, which helps the metals to achieve a formal electronic saturation. In this case, the limited propensity of the complex toward any CO substitution reactions is likely attributable to the hindered mobility of the (CO)(2)Rh(I) fragment.

Abstract: Cationic complexes [MoCp(2)(U)][PF6] (1), [MCp(2)(2TU)][PF6] (2, 2'), [MCp(2)(4TU)][PF6] (3, 3'), [MoCp(2)(2,4-DTU)]Cl (4), [MCp(2)(2MTU)][PF6] (5, 5') and [MCp(2)(4MTU)][PF6] (6, 6') (M = Mo, W; Cp = eta(5)-C5H5; U = uracilato; 2TU = 2-thiouracilato; 4TU = 4-thiouracilato; 2,4-DTU = 2,4-dithiouracilato; 2MTU = 2-methylthhiouracilato and 4MTU = 4-methylthiouracilato) have been prepared from the reaction of [MCp(2)Cl(2)] with the corresponding uracil in the presence of triethylamine. All the uracilato derivatives act as bidentate ligands and experimental evidence CX-ray and IR) shows the following preference for binding atoms in this system: N(3) > N(1), = S > = O, O(2) > O(4); although there is no clear evidence it is also likely that S(2) > S(4). Electrochemical studies, by cyclic voltammetry carried out in acetonitrile or dimethylformamide, showed that compounds where ligands coordinate through N,S atoms are more stable upon oxidation than compounds where ligands coordinate through N,O donor atoms. The molecular structures of [MoCp(2)(2TU)][PF6] (2), [WCp(2)(2TU)][PF6] (2'), [MoCp(2)(4TU)][PF6] (3), [MoCp(2)(2MTU)][PF6] (5), [WCp(2)(2MTU)][PF6] (5') and [MoCp(2)(4MTU)][PF6] (6) have been determined by X-ray crystallography.

Abstract: The recently reported dodecamer [Co-12(mu(3)-S)(14)(mu(4)-S)(2)(PEt(3))(10)](2+) is envisaged as the condensation product between two octahedral [Co-6(mu(3)-S)(8)(PEt(3))(6)](+) clusters upon loss of one terminal phosphine ligand each. The latter, with 97 valence electrons, has the same primary geometry as several other species with skeleton M(6)(mu(3)-X)(8) and electron count in the range 80-98. Depending on the general MO architecture of these clusters, the unpaired electron in [Co-6(mu(3)S)(8)(PEt(3))(6)](+) is highly delocalized, and the loss of one terminal ligand does not affect this situation. A visual MO analysis, performed at the extended Huckel level, highlights the perturbation induced at one hexameric unit upon approaching of an equivalent unit. Even at long separations, the mixing between two members of the original set 1t(1u) (O-h symmetry) localizes the unpaired electron largely at the metal which is involved in the incipient Co-S exo linking. Eventually, the condensation of the two units is strongly favoured by the spin coupling between two metal radicals. The exo Co-Co linkage has bond order much closer to 1 than all of the other intermetallic linkages.

Abstract: The molecular structure of trans-[(NH3)(2)Pt(mu-1-MeC(-))(2)Pd(NH3)][Pd(NH3)(4)](0.5)(NO)3 . 3H(2)O (1) has been determined by X-ray crystallography. The compound crystallizes in the triclinic space group P $($) over bar$$ 1 (No. 2), with a = 7.476(2) Angstrom, b = 11.373(3) Angstrom, c = 16.397(4) Angstrom, alpha 87.27(2)degrees, beta = 85.32(2)degrees, gamma = 88.74(2)degrees, and Z = 2. The structure was refined to R = 0.034 and R(w) = 0.038 for 5045 independent reflections. The complex is best described by a square planar coordination of Pd and a square pyramidal one of Pt with the Pd in the apical position (T over square geometry, TSQ). The Pt and Pd atoms are bridged by two nearly coplanar 1-methylcytosinate anions with a Pt-Pd distance of 2.511(1) Angstrom. A theoretical analysis, based on EHMO calculations, highlights the nature of the single M-M' bond in d(8)-d(8) dimers of this type (4:3) and points out the correlations with the better known d(8)-d(8) dimers (4:4) characterized by two parallel square planar coordination geometries. The M-M' bond order in the latter is close to zero, but not null. Finally, the fate of the M-M' linkage for adding one or two electrons to the above TSQ species is formally analyzed. A correlation is made with the structural data available for dimers with d(8)-d(9) and d(8)-d(9) electron counts [e.g. Pt(II)-Cu(II) and Pt(II)-Hg(II) species with bridging nucleobases]. While one additional electron weakens the M-M' linkage without destroying the primary TSQ geometry, two extra electrons force the expulsion of the ligand formerly coaxial with the metals (4:2 adducts). The M-M' interaction in the 4:2 adducts is comparable with that proposed for the 4:4 species.

Abstract: eta3-Benzyl palladium complexes have been prepared by insertion of styrene in the Pd-Me bond of the solvated cation [L2PdMe(solv)]+ (L2 = chelating ligand). The eta3 coordination of the benzyl ligand has been elucidated by X-ray structure determination and NMR spectroscopy. NMR studies have shown that [(eta3-R-alpha-benzylPdL2]+ (R = Me or Et) undergo two dynamic processes in solution: eta3 - eta1 coordination of the benzyl group and ligand exchange of coordination sites. The TMEDA and DIPY complexes undergo a rapid and reversible reaction with carbon monoxide to yield (R-alpha-benzylCO)PdL2CO]+.

Abstract: The X-my crystal structure analyses of the three redoX pairs [Rh2(CO)2LL'{mu-PhNC(Me)NPh}2]z [z = 0, 1; L = L' = PPh3 (1, 1+); L - L' = P(OPh)3 (2, 2+); L = PPh3, L' - P(OPh)3 (3, 3+)] are reported and the molecular structures compared with those Of [Rh2(CO)2(PPh3)2(mu-RNNNR)2]z [z - 0, 1; R = p-tolyl (4,4+)]. These have locally planar rhodium coordination with the planes splayed apart between 20 and 40-degrees. The, non Rh...Rh distance of ca. 2.9 angstrom in the neutral complexes is reduced by between 0.14(2) and 0.262(4) angstrom on oxidation. These and other changes in the molecular structure arc consistent with the HOMO of the neutral complexes being delocalized, over the two metals, and having rhodium-rhodium sigma* character. ESR spectroscopic analysis is reported on 1+-4+ and on the related ligand-bridged [Rh2]3+ species [Rh2(CO)2(mu-dPPM)(mu-RNNR)2]+ [R = p-tolyl (5+)], [Rh2(mu-diMen)2(mu-dppM)2]3+ (6+, dimen = 1,8-diisocyanomenthane), [Rh2(mu-TM4)2(mu-dppM)2]3+ [TM4 = 2,5-diisocyano-2,5-dimethylhexane (7+)], [Rh2(mu-CNCH2CH2CH2NC)2(mu-dppm)2]3+ (8+), [Rh2(mu-HTP5)2(mu-dppm)2]3+ (9+,HTP5 - 1,5-diisocyano-1,1,5-triphenylpentane), [Rh2(CO)2(PMe2Ph)2(mu-chp)2]+(10+), [Rh2(mu-mhp)2(COD)2]+ (11+). [Rh2(mu-ChP)2(NBD)2]+ (12+), and [Rh2(mu-mhp)2(NBD)2]+ (13+). The spectroscoPiC parameters are shown to be dependent on the geometry of the cations and especially on the splay angle between the rhodium coordination planes. Quantitative interpretation of the ESR parameters suggests an orbital composition for the SOMO which is in good agreement with extended Huckel MO calculations on the model compounds [{Rh(H)2(CO)2}2]2- (14) and [Rh2(CO)4(mu-HNNNH)2] (15). The theoretical and ESR analyses are in full accord with the X-ray results and together provide a coherent picture of the electronic and geometric structures of ligand-bridged [Rh2]3+ complexes.

Abstract: The title structure, chloro{2-(diphenylphosphino)-benzaldehyde-P}[N-12-(diphenylphosphino)ben zyli-dene}ethylamine-N,P](3,4,5,6-tetrachloro-1,2-benzenediolato-O,O')ir idium butanol hemisolvate, [IrCl(C6Cl4O2)(C19H15OP)(C21H20NP)].0.5C4H10O, consists of two independent molecules in the asymmetric unit with an Ir(III) metal atom pseudo octahedrally coordinated by the following donors: two catecholate 0 atoms, one Cl- anion, two phosphine groups and one imino group.

Abstract: The highly delocalized distribution of the MOs in C60 has been analyzed with the aid of extended Huckel calculations and their graphic representations. Symmetry and perturbation theory arguments rationalize the MO correlations in terms of the implosion of 12 C5 rings distributed at the vertices of an expanded icosahedron. In particular, two distinguishable sigma and pi C60 subsets are each split into 30 filed and 30 empty MOs belonging to the same species under I(h) symmetry. As a major difference, whereas 30 new C-C sigma bonds add to the 60 pre-existing C5 endocyclic bonds, some partial pi bonding at the 6:6 edges of C60 occurs at the expense of pi bonding at the 5:6 edges. This point has been clarified by referring to the electronic structure of the simpler molecule [5]-radialene which typifies the MO response to the idea of pi resonance. An overview at the C-C bonding/antibonding roles in all of the pi MOs simplifies understanding their possible interactions with the frontier MOs of typical transition metal fragments. The model complexes considered involve the eta2-coordination of the C2v fragment (PH3)2Pt to C60 in 6:1, 2:1 and 1:1 ratios. The MO analysis highlights the electronic causes of the deformation from sphericity of C60 when a metal is attached to one or more 6:6 edge. Attempts to explore alternative, but as yet unsynthesized coordination modes have also been made.

Abstract: The iodide-abstraction reaction of [{Rh2(mu-I)(CO)(bipy)(mu-RNNNR)2}2][PF6]2 1 (bipy = 2,2'-bipyridyl, R = p-tolyl) with AgPF6 in MeCN gave [Rh2(CO)(NCMe)2(bipy)(mu-RNNNR)2][PF6]2 2 which slowly decarbonylated at room temperature to [Rh2(NCMe)3(bipy)(mu-RNNNR)2][PF6]2 3. The crystal structure to 3 shows as Rh-Rh single bond [2.534(2) angstrom] and one of the three terminal nitrile ligands axially attached to the bipy-bound rhodium atom. Complex 3 reacts with Na[S2CNMe2].2H2O to give [Rh2-(NCMe)(S2CNMe2)(bipy)(mu-RNNNR)2][PF6] 4. In CH2Cl2, the iodide-abstraction reaction of 1 affords a green solution containing a carbonyl complex [Rh2(CO)(solv)2(bipy)(mu-RNNNR)2]2+ A (solv = CH2Cl2), an analogue of 2. Complex A (solv = CH2Cl2) reacted with neutral chelating ligands to give the carbonyl-bridged complexes [Rh2(mu-CO)(L-L)(bipy)(mu-RNNNR)2][PF6]2 [L-L = bipy 5, 4,4'-dimethyl-2,2'-bipyridyl (dmbipy) 6, 1,10-phenanthroline (phen) 7, di-2-pyridylamine (dpa) 8, or Ph2PCH2CH2PPh2 (dppe) 9] the last of which undergoes reduction with NaBH4 to give paramagnetic [Rh2(CO)(dppe-P)(bipy)(mu-RNNNR)2][PF6] 11 having a face-to-face structure with a monodentate dppe ligand. With Ph2PCH2PPh2 (dppm), A (solv = CH2Cl2) afforded [Rh2(mu-CO)(dppm)(bipy)(mu-RNNNR)2][PF6]2 10a X-ray studies on which, as a CH2Cl2 solvate, reveal a carbonyl-bridged open-book structure with a Rh-Rh distance of 3.179(2) angstrom and a large Rh-C(O)-Rh angle of 108.3-degrees. Complex 10a equilibrates with the face-to-face, terminal carbonyl isomer [Rh2(CO)(dppm)(bipy)(mu-RNNNR)2][PF6]2 10b in solution. The electronic structures of the two isomers have been probed by extended-Huckel molecular-orbital calculations on the model compound [Rh2H4(CO5]. These show (i) the absence of metal-metal bonding in 10a while 10b has a Rh-Rh sigma bond, and (ii) that the CO in 10a is best viewed as more ketonic than a typical bridging carbonyl. The reaction of complex A (solv = CH2Cl2) with neutral chelating ligands also gives low yields of [{Rh2(CO)(O2PF2)(mu-O2PF2)(bipy)(mu-RNNNR)2}2] the crystal structure of which shows two dirhodium fragments [Rh-Rh 2.505(4) angstrom] linked by two O2PF2 groups bridging across axial and equatorial sites in different [Rh2]4+ moeties. The reaction of A (solv = CH2Cl2) with N-SH ligands yields [Rh2(CO)(N-S)(bipy)(mu-RNNNR)2][PF6] (N-S = 1-methyl-2-sulfanylimidazolate 13, 2-sulfanylpyrimidinate 14, 2-sulfanylthiazolinate 15, or 2-sulfanylbenzimidazolate 16), and NaX gives [Rh2(CO)X2(bipy)(mu-RNNNR)2] (X = Cl or NO2).

Abstract: It is usually stated that the two iron atoms in Fe2(CO)9 share a direct bond. A theoretical analysis, however, shows that this matter is more complex. By populating or depopulating certain frontier orbitals of Fe2(CO)9 and optimizing the corresponding hypothetical systems one can split up the interactions, the sum of which leads to the overall structure of the Fe2(CO)9 complex. The different interactions between the complex fragments can be connected with certain molecular orbitals and identified with respect to their bonding or antibonding character. It turns out that only a small direct Fe-Fe attractive interaction can exist, which is, however, hidden by a relatively large effective Fe-Fe repulsion. Consequently, the bioctahedron is linked together through iron-bridge bonds.

Abstract:

Abstract:

Abstract: In many cases, the decomposition of methyltrioxorhenium (CH3ReO3, MTO) and other alkyl and aryl derivatives appears occur through the homolytic cleavage of the Re-C bond. Under these circumstances, the already deficient metal [Re(VII)] gives up half of the electron pair donated by the alkyl anion, and the ensuing Re(VI) species has a finite lifetime as a free radical. The problem is tackled from the viewpoint of qualitative MO theory. From a detailed examination of all the orbital interactions between metal and the coordinated atoms in pseudotetrahedral MTO, it is established that the metal receives up to nine electron pairs from the oxygen atoms and CH3. Accordingly, the complex is better formulated as an 18- rather than the widely accepted 14-electron species. The study of the dynamics of the most critical frontier MOs on elongating the Re-C bond provides significant chemical information. Particularly relevant for the one-electron transfer from carbon to the metal atom is the avoided crossing between the HOMO and the LUMO. With the flattening of both separating fragments taken into account, the two levels converge to the accidentally quasi-degenerate sigma-CH3 and ReO3 z2 orbitals. In the model, the effects of substituting groups such as phenyl or tert-butyl for methyl are also pointed out.

Abstract: The reaction of lithium enolates with chlorodiphenylphosphine provides a convenient large scale synthesis of beta-keto phosphines. The coordination of beta-keto phosphines 1 at an (eta6-arene)Cl2Ru fragment affords neutral complexes (eta6-arene)Cl2Ru[eta1-P-Ph2PCHR2C(=O)R1], 2, cationic complexes {(eta6-arene)ClRu[eta2-P,O-Ph2PCR3R2C(R1)=O]}+, 3, and neutral enolato derivatives (eta6-arene)ClRu[eta2-P,O-OC(R1)=C(R2)PPh2],4. Compounds 4 react under mild conditions with ligands L (L = Me2S or RC=N) to give cationic derivatives {(eta6-arene)(L)ClRu[OC(R1)=C(R2)PPh2]}+, 5. When L is PhC=CH, carbon-carbon bond formation leads via regioselective coupling of the C=C bond to both ruthenium and =C(PPh2) carbon to complexes {(eta6-arene)Ru[eta3-CH=C(Ph)C(R2)(PPh2)C(R2)=O]}+, 6. The crystal structures of the compounds 6a (arene = p-cymene, R2 = H, R1 = Bu(t)), monoclinic, space group P2(1)/n (No. 14) with Z = 4, a = 14.870(4) angstrom, b = 20.087(6) angstrom, c = 12.454(5) angstrom, alpha = 90.0-degrees, beta = 92.67(3)degrees, gamma = 90.0-degrees, d(calcd) = 1.37 g/cm3, and 6c (arene = mesitylene, R2 = Me, R1 = Et), triclinic, space group P1BAR (No. 2) with Z = 2, a = 10.962(1) angstrom, b = 15.473(2) angstrom, c = 10.910(1) angstrom, alpha = 100.10(1)degrees, beta = 113.64(1)degrees, gamma = 102.41(1)degrees, d(calcd) = 1.55 g/cm3, have been determined. 6b (arene = mesitylene, R2 = H, R1 = Bu(t)) is cleanly thermally converted to the metallaphosphacyclopropane derivative {Ru[eta3-CH(PPh2)CPh=C(H)C(But)=O] (mesitylene)}+, 7, as shown by the X-ray structure determination: triclinic, space group P1BAR (No. 2) with Z = 2, a = 14.034(2) angstrom, b = 11.944(1) angstrom, c = 10.957(1) angstrom, alpha = 106.56-degrees, beta = 98.34(1)degrees, gamma = 109.24(1)degrees, d(calcd) = 1.44 g/cm3.

Abstract: Tin-119 NMR data have been used to examine the effective coordination spheres in dichloromethane solution of a series of organoyltin(IV) 1,1-dithiolate compounds RSnX(n)(S-S)3-n (where R=Ph, Me; X=Cl, Br; n=0, 1, 2), for S-S=S2CNEt, S2COEt, S2P(OEt)2. Intermolecular balide and 1,1-dithiolate exchange are slow on the NMR timescale however the tin-119 chemical shifts are temperature dependent due to the intramolecular equilibrium between monodentate and bidentate coordinated 1,1-dithiolate ligand. The magnitude of the temperature dependence of the tin-119 chemical shifts for the series RSn(S-S)3 indicates that the order of 1,1-dithiolate ligand donor strength is Et2dtc>Etxan>Et2dtp. Mixed ligand complexes PhSn(S-S)(S-S)2' may be formed either by redistribution reactions of PhSn(S-S)3 and PhSn(S-S)3' or by addition of 1,1-dithiolate salt to solutions of PhSnCl(n)(S-S)3-n (n=1,2). The crystal structure of PhSn(S2CNEt2)3 has been determined. The geometry about tin in PhSn(S2CNEt2)3 is a distorted pentagonal bipyramid with tin-sulfur distances ranging from 2.487(l) to 2.794(2) angstrom.

Abstract: The reaction of ketene silyl acetals with [(eta3-allyl)PtL2]+OAc- leads to formation of platina-(II)cyclobutanes upon the nucleophilic attack of the enolate at the central allyl carbon atom (C(c)). As a result, from a new theoretical analysis for complexes of the type [(eta3-allyl)ML2]+ (M = Pd, Pt), the C(c)-centered MO (C3-pi(perpendicular-to)*) is not destabilized as usually assumed. Rather, it competes with another MO, (d(pi)-nC3) (centered on metal and terminal C(t) carbon atoms), to be the first LUMO of the system. On this basis, the attack of a nucleophile at the C(c) atom is almost as probable as attack at the C(t) atoms. The initial MO picture of the precursor complex can only suggest the regioselectivity of the reaction, saying nothing about the attractive/repulsive interactions encountered by the incoming nucleophile in the pathway toward metallacyclobutane. Within the limits imposed by the EHMO method, a simple qualitative description is offered of the intended correlations among metal-carbon and carbon-nucleophile bonding/antibonding MO's during the formation of the metallacyclobutane product. An energetic barrier, which varies as a function of the sigma-donor strength of the nucleophile, is observed, and its origin is described.

Abstract: The coordinated allyl group of complexes [(eta3-allyl)ML2]+ (M = Pd or Pt) is shown experimentally to undergo nucleophilic attack on either central (C(c)) or terminal (C(t)) allyl carbon atoms. A new theoretical discussion is presented, showing that the feasibility of attack at the C(c) atom is not as low as previously thought.

Abstract: A comparison between the structures of the two pseudo-trigonal bipyramidal complexes [Mn(N(CH2CH2PPh2)3I]BPh4, 1 and [Mn(N(CH2CH2NMe2)3Br]BPh4. 2, has shown that, notwithstanding the unique Mn(II) high spin electron configuration, the Mn-Na, bond is significantly stretched [2.19(3) vs. 2.634(9) angstrom] when three equatorial amine donors are substituted by three phosphine analogs. A rationalization can be provided by a qualitative MO analysis, based on EHMO calculations. The metal d-orbitals (all half-filled) have no major role in the formation of any Mn-L bond and the metal uses only its four s and p orbitals to ensure as many as five bonds in 2. By contrast, such an hypervalency is strongly reduced in 1, as almost 100 % of metal s orbital is engaged in bonding-antibonding interactions with the equatorial phosphine lone pairs. Accordingly, the residual possibility for the metal to coordinate as many as two axial ligands is minimized.

Abstract: The complexes [IrH2(eta1S-Th)2(PPh3)2]PF6 (Th = thiophene (2), benzo[b]thiophene (3), dibenzothiophene (4) and tetrahydrothiophene (5)) have been synthesized in high yields by hydrogenation of [Ir(COD)(PPh3)2]PF6 (COD = 1,5-cyclooctadiene) in the presence of the appropriate thiophene. The structures of 2 and 5 have been determined by X-ray diffraction. Crystal data: 2, monoclinic, space group C2/c, with a = 12.742 (6) angstrom, b = 17.078 (4) angstrom, c = 22.920 (3) angstrom, beta = 93.33 (2)-degrees, Z = 4, and R = 0.038 (R(w) = 0.041) for 2802 unique reflections; 5, monoclinic, space group P2(1)/a with a = 21.995 (5) angstrom, b = 19.428 (3) angstrom, c = 10.403 (5) angstrom, beta = 100.59 (2)-degrees, Z = 4, and R = 0.064 (R(w) = 0.071) for 4058 unique reflections. In both complexes the coordination geometry around the metal atom consists of a distorted octahedron with mutually cis S-bonded thiophenes, cis hydrides and trans triphenylphosphines. These structures can be considered as models for the initial chemisorption step in the hydrodesulfurization of thiophenes on solid catalysts.

Abstract: The paper presents a qualitative MO analysis of highly symmetrical, mostly diamagnetic octahedral metal clusters. Two classic types are considered: (i) clusters supported by face-capping ligands with general formula M(6)(mu(3)-X)(8)L(6) (X=pi donor or acceptor, L=two-electron sigma donor or the cyclopentadienyl anion) and a variety of electron counts; (ii) clusters with only terminal ligands, e.g. [M(6)(CO)(18)](2-) (M=Ru, Os), Ni(6)Cp(6) and the heteronuclear Ni(2)Zn(4)Cp(6), uniquely characterized by a trans-octahedron Ni-Ni bond. Rather than reporting another series of numerical results for compounds already widely investigated, the article surveys the MO architectures in the attempt to schematize a few distinctive roles for all the levels. The specific orbital contributions to the intraligand or metal-ligand or metal-metal bonding networks are determined. Almost in all of the cases, the MOs having M-M bonding/antibonding character comply with a general scheme made of interpenetrating radial, tangential, d delta atomic orbitals. Importantly, the availability of d delta orbitals allows, at least in cases such as the 84e species [Mo-6(mu(3)-Cl)(8)Cl-6](2-), a full correspondence between the numbers of M-M bonds and octahedral edges to be drawn. The metals need not be attributed the hypervalence which is implicit in theories (such as PSEPT) based on the analogies between metals and main group elements (i.e. only radial and tangential orbitals are involved in the octahedral skeleton bonding). It is shown that different electron populations also adapt to a generalized scheme of the interactions but that the effects on M-M bonding vary from case to case. Most of the interpretative work is empirically done with the visual aid provided by the program CACAO, which allows interactive graphing of numerical EHMO results.

Abstract: The cis hydride chloride complex [(PP3)RuH(Cl)] (1) [PP3 = P(CH2CH2PPh2)3] in CH2Cl2 is capable of extracting Tl+ from TlPF6 in the aqueous phase to give [(PP3)RuH(ClTl)]PF6 (2), which contains a unique example of eta1-(Cl)-TlCl ligand. Compound 2 decomposes in refluxing THF, separating TlCl and forming the unsaturated [(PP3)RuH]+ moiety, which, by addition of PPNCl, restores the starting chloride 1 quantitatively. The latter complex is capable of transporting Tl+ from one aqueous solution through a dichloromethane solution into a second aqueous phase (U-tube experiment).

Abstract: O-17 NMR spectra (natural abundance) for selected acylsilanes show high-frequency chemical shifts with respect to the corresponding ketones. An MO qualitative analysis clarifies the origin of the above experimental finding as being due to other well-known chemical and physical differences between the two types of compound. The importance of a relatively small HOMO-LUMO gap in acylsilanes is stressed. The alkyl and silyl substituents promote different electron distributions over the CO-sigma and pi-fragment orbitals which can ultimately contribute to the shift of the NMR resonances. The MO analysis addresses new viewpoints on acylsilanes: (i) the silyl groups, rather than strong sigma-donors should be better defined as weak sigma-acceptors; (ii) the carbonyl carbon atom is negatively charged, but the electron density is accumulated in the sigma-orbitals and not in the p(pi) one; (iii) the IR data are rationalized in terms of a weakened in plane component of the CO bond, while the pi(perpendicular-to), bond is unaffected; (iv) the carbonyl carbon atom keeps its electrophilic character in acylsilanes and it can exert nucleophilicity only upon departure of SiR3+, a good leaving group.

Abstract: A large number of five-coordinate metal catecholate complexes of the general formula [(triphos)M(Cat)]Y have been synthesized and characterized by chemical, spectroscopic and electrochemical techniques (M=Co, Rh, Ir; Cat=9,10-phenathrenecatecholate, 1,2-naphthalencatecholate, 3,5-di-tert-butylcatecholate, 4-methylcatecholate, 4-carboxycatecholate-ethylester, tetrachlorocatecholate; Y=BPh4, PF6; triphos=MeC(CH2PPh2)3). All of the compounds undergo electron-transfer reactions that encompass the M(III), M(II) and M(I) oxidation states of the metal, and the catecholate, semiquinone and quinone oxidation levels of the quinoid ligand. Paramagnetic Ir(III) semiquinonate complexes, [(triphos)Ir(SQ)]2+, and Ir(II) catecholates, [(triphos)Ir(Cat)], have been characterized by X-band ESR spectroscopy. The reactions of the metal catecholates in non-aqueous media with dioxygen have been investigated. With very few exceptions, all of the compounds react with O2 to give adducts of the general formula [(triphos)M(OO)(SQ)]Y. An X-ray analysis bas been carried out on [(triphos)Ir(OO)(PhenSQ)]BPh4 (Phen=9,10-phenanthrenesemiquinonate). In the complex cation, the metal is octahedrally coordinated by the three phosphorus atoms of triphos and by three oxygen atoms, one from O2 and the other two from the catecholate ligand that has attained a semiquinoid character. The electrochemical behavior of the dioxygen adducts has been studied in detail. Depending on the E-degrees' values relative to the M(III)(SQ)/M(III)(Cat) couples of the parent metal catecholates, the dioxygen adducts undergo either a one-electron oxidation to give o-quinone complexes [(triphos)M(Q)]3+ and superoxide ion (O2-) or a two-electron oxidation to give [(triphos)M(Q)]3+ and O2. Several factors have been found to affect the O2 uptake by metal catecholates. Of particular importance are: (i) the coordination number of the metal; (ii) the basicity of either the catecholate ligand or metal; (iii) the temperature; (iv) the pressure of dioxygen. The role of each factor has been analyzed and rationalized. The transport of dioxygen from one metal catecholate to another has been studied. A mechanistic interpretation for the formation of the [(triphos)M(OO)(SQ)]+ complexes is proposed in light of a large crop of experimental data and molecular orbital considerations.

Abstract: The addition of 2-(diphenylphosphino)pyridine (NC5H4PPh2-2) to an in situ prepared m-xylene solution of [Mo2(eta-C5H5)2(CO)4] (molar ratio 2:1 ), at room temperature, yields the complex [Mo2(eta-C5H5)2(CO)4(NC5H4PPh2-2)2] 1. Compound 1 is easily converted into the cationic molybdenum(II) mononuclear complex [Mo(eta-C5H5)(CO)3(NC5H4PPh2-2)]PF6 2 by reaction with AgPF6. Reduction of 1 with Na or Na/Hg, in tetrahydrofuran, affords an air-sensitive solution containing Na[Mo(eta-C5H5)(CO)2(NC5H4PPh2-2)] 3, together with minor products. Electrochemical measurements show that 1 undergoes a reversible one-electron oxidation followed by relatively slow decomposition of the electrogenerated species. The molecular structure of the diethyl ether solvate of 1 was determined by X-ray diffraction methods: monoclinic, space group P2(1)/c, with a = 13.218(4), b = 19.485(6), c = 11.01 9(4) angstrom, beta = 110.10(2)-degrees and Z = 2. The centrosymmetric complex 1 is a typical piano-stool dimer in which two units share the leg coinciding with the Mo-Mo vector. Similarly to other compounds of this type, the M-M separation is quite long [Mo-Mo 3.276(3) angstrom], ca. 0.5 A longer than the sum of the metal radii. The evidence for the single metal-metal bond (predicted by counting rules) and its role in providing the system's stability is discussed in terms of qualitative molecular orbital theory. The extended-Huckel method used appears sufficiently reliable as the total electronic energy minimizes for an intermetal separation close to the experimental one. The loss of sigma bonding, on elongating Mo-Mo, is counterbalanced by the diminished repulsion between metal lone pairs (filled xy orbitals), thus the intermetallic distance is an evident compromise between attractive and repulsive electronic forces. Steric factors may not be so important. The theoretical implications for the eventual homolytic cleavage of the dimer are also underlined. A rationale is provided for the effects that follow the removal of one electron from the system.

Abstract: Treatment of [{Rh2I(CO)(bipy)(mu-RNNNR)2}2][PF6]2 2 with AgPF6 and dppm gives [Rh2(mu-CO)(bipy)(L-L)(mu-RNNNR)2]2+ (5; L-L = dppm), X-ray and MO studies on which show that it may be viewed as an open-book [Rh2]6+ core bridged by a 'ketonic' CO2- ligand; this isomer is in equilibrium in solution with a face-to-face isomer in which the carbonyl ligand is terminally boLnd to one metal only, a related open-book to face-to-face conversion occurring on one-electron reduction of (5; L-L = dppe) to [Rh2(CO)(bipy)(dppe-p)(mu-RNNNR)2]+7.

Abstract: 1,8-Naphthyridinium tetrachloroferrate(III)-1,8-naphthyridine (1/1), C8H7N2+.FeCl4-.-C8H6N2, M(r) = 458.9, monoclinic, Cc, a = 10.698 (2), b = 25.533 (4), c = 7.315 (2) angstrom, beta = 101.83 (1)-degrees, V = 1955.67 (72) angstrom 3, Z = 4, D(x) = 1.5588 Mg m-3, lambda(Cu K-alpha) = 1.5418 angstrom, mu = 11.4 mm-1, F(000) = 924, room temperature, final R = 0.0704 for 929 reflections. The structure consists of discrete tetrahedral FeCl4- anions, protonated (NNH)+ and unprotonated (NN) 1,8-naphthyridine molecules.

Abstract:

Abstract: In the course of studies aimed at developing new catalytic systems, we have explored the possibilities offered by the modification of thiolato bridges in dinuclear rhodium complexes, together with the influence of the cocatalyst, on the selective hydroformylation of cyclic vinyl and allyl ethers. The dinuclear complex [Rh2(mu-S(CH2)3NMe2)2(cod)2] (Cod = 1,5-cyclooctadiene) has been prepared, and its reactivity with CO, phosphines, and phosphites has been investigated. The complex crystallizes m the monoclinic space group C2/c with Z = 8, a = 22.543 (4) angstrom, b = 12.040 (2) angstrom, c = 21.547 (3) angstrom, and beta = 98.77 (1)-degrees. For the determination of the structure 4091 unique reflections were used, and the final refinement gave R = 4.1% and R(w) = 4A%. The molecular structure reveals that the two rhodium atoms are bridged by the two thiolato ligands, and the cyclooctadiene completes the coordination of the metal atoms. The amine groups are not bonded to the rhodium. The dinuclear complex has been used in the hydroformylation of 2,3-dihydrofuran, 2,5-dihydrofuran, 3,4-dihydro-2H-pyran, and 3,6-dihydro-2H-pyran. Hydroformylation reactions of dihydrofurans required conditions milder than those for dihydropyrans. The major product in the hydroformylation of 3,4-dihydro-2H-pyran or 3,6-dihydro-2H-pyran was tetrahydropyran-2-carbaldehyde. A systematic study of the influence of the reaction parameters on the selectivity of the hydroformylation of 2,3-dihydrofuran and 2,5-dihydrofuran was undertaken. The study allowed the rationalization of the observed selectivity and the optimization of the yields and regioselectivities. Thus, by modification of the reaction parameters, tetrahydrofuran-3-carbaldehyde was obtained in quantitative yields from 2,5-dihydrofuran and tetrahydrofuran-2-carbaldehyde can be prepared from either 2,3-dihydrofuran or 2,5-dihydrofuran in approximately 75% yield.

Abstract: The reaction of anions [Fe2(mu-CO)(CO)6(mu-PR2)]- with R'2PCl gives either the electron-precise bis-(phosphido)-bridged diiron hexacarbonyl complexes [Fe2(CO)6(mu-PR2)(mu-PR'2)] (R = Ph, R' = Cy (1), Bu(t) (3); R = R' = Cy (2); R = Me, R' = Bu(t) (4)) or the new electron-deficient diiron pentacarbonyl complex [Fe2(CO)5(mu-PBu(t)2)2] (5). Thermal treatment of 3 affords another complex of this latter type, [Fe2(CO)5(mu-PBu(t)2)(mu-PPh2)] (6). The structures of the three bis(phosphido)-bridged diiron complexes [Fe2(CO)6(mu-PBu(t)2)(mu-PPh2)] (3), [Fe2(CO)5(mu-PBu(t)2)(mu-PPh2)] (6), and [Fe2(CO)5(mu-PBu(t)2)2] (5) have been determined by X-ray analysis. Crystal data for 3: monoclinic, space group P2(1)/c, a = 10.143 (4) angstrom, b = 16.854 (8) angstrom, c = 16.871 (7) angstrom, beta = 105.37 (3)-degrees, Z = 4, R (R(w)) = 0.035 (0.034). Crystal data for 6 at -95-degrees-C: monoclinic, P2(1)/c, a = 14.091 (7) angstrom, b = 12.090 (5) angstrom, c = 16.813 (7) angstrom, beta = 112.60 (4)-degrees, Z = 4, R (R(w)) = 0.026 (0.033). Crystal data for 5: triclinic, P1BAR, a = 8.642 (5) angstrom, b = 11.730 (7) angstrom, c = 13.648 (9) angstrom, alpha = 76.61 (3)-degrees, beta = 78.76 (3)-degrees, gamma = 76.40 (4)-degrees, Z = 2, R (R(w)) = 0.033 (0.035). The electron-precise complex 3 has a strongly folded Fe2P2 core with a flap angle theta = 117.1-degrees and an Fe-Fe distance of 2.707 (1) angstrom, whereas the two electron-deficient complexes 6 and 5 have only weakly folded or essentially planar Fe2P2 cores, respectively, and considerably shorter Fe-Fe distances (6, theta = 166.0-degrees, Fe-Fe = 2.462 (1) angstrom; 5, theta = 176.2-degrees, Fe-Fe = 2.484 (2) angstrom). EHMO calculations confirm that the short iron-iron bond distances in the bis(phosphido)-bridged diiron pentacarbonyl complexes are consistent with the presence of iron-iron double bonds.

Abstract:

Abstract: [Fe(C5H5O2)(C42H42P4)][B(C6H5)4], M(r) = 1142.9, monoclinic, P2(1)/a, a = 26.296 (3), b = 12.304 (4), c = 19.061 (2) angstrom, beta = 103.63 (2)-degrees, V = 5993.4 (22) angstrom3, Z = 4, lambda(Cu Kalpha) = 1.5418 angstrom, mu = 33.9 cm-1, F(000) = 2404, room temperature, final R = 0.086 for 4321 significant reflections. The molecular structure consists of tetraphenylborate anions and discrete complex cations in which an Fe(II) metal is pseudo-octahedrally coordinated by the ligand pp3 [pP3 = P(CH2CH2PPh2)3] and by a 4-pentynoate group. The latter ligand is bonded in a chelating mode through the two 0 atoms.

Abstract: Tin-119, phosphorus-31, and carbon-13 NMR data have been used to examine the effective coordination spheres in dichloromethane solution of a series of triorganoyl- and diorganoyltin(IV) dithiolate compounds R3Sn(S-S) (where R = Ph, Me) and R2Sn(S-S)2 (where R = Ph, Me, (n)Bu, (t)Bu) for S-S = S2CNEt2, S2COEt, and S2P(OEt)2 as well as diorganoyltin(IV) dithiolate halide complexes R2SnX(S-S) (R = Ph; X = Cl, Br; and R = Me, (n)Bu, (t)Bu; X = Cl). For R3Sn(S-S) compounds only dithiocarbamate (S-S = S2CNEt2) Seems to be have as an actual bidentate sulfur ligand whereas for other systems intramolecular monodentate-bidentate dithiolate exchange is rapid at room temperature, with the equilibrium favoring bidentate sulfur coordination at low temperature for some systems. The effectiveness of dithiolates as chelate ligands becomes more evident in R2Sn(S-S)2 and in R2SnCl(S-S) derivatives. There is evidence that the dithiocarbamate ligand in Ph2SnCl(S2CNEt2) is bidentate and that the compound is five coordinate and stereochemically rigid in solution at -100-degrees-C. A comparison of solid-state and solution NMR data indicates that in solution the dithiocarbamate ligand acts as a bidentate sulfur donor in Me2Sn(S2CNEt2)2 but only as a monodentate donor in (t)Bu2Sn(S2CNEt2)2. The crystal structure of (t)Bu2Sn(S2CNEt2)2 has been determined and is almost identical to the known structure of Me2Sn(S2CNEt2)2. In the structure of (t)Bu2Sn(S2CNEt2)2 the Sn-S bond lengths are asymmetric (2.5 and 2.9 angstrom), but the overall stereochemistry suggests that the role of the two sulfur atoms of each chelate may be easily interchanged, possibly through a pseudooctahedral intermediate having the chelates symmetrically displaced. There is no evidence for any steric stress resulting from the (t)Bu groups, and consequently electronic influences are invoked to explain observed differences in stereochemical nonrigidity for various hypervalent tin(IV) systems in solution. The crystal structures of (t)Bu2SnCl(S2CNEt2) and Ph2SnCl(S2CNEt2) have also been determined, and in each case there is asymmetric coordination by the sulfur atoms of the dithiolate ligand to tin.

Abstract:

Abstract: Salts of the anion [Co(COOCH3)2(CO)3]- with Cs+ and K+ (with the latter cation complexed by dibenzo-18-crown-6 ether) have been isolated and structurally characterized. In the trigonal bipyramidal cobaltate two C-coordinated COOCH3 groups occupy trans-axial positions. There are ionic interactions between the cation and the terminal oxygen atoms of methoxycarbonyl involving both of the coordinated COOCH3 groups in the case of the Cs salt but only one of them in the case of the K salt. One methoxy group of the anion [Co(COOCH3)2(CO)3]- is strongly nucleophilic, as shown by the reactions with Co2(CO)8 or CO2. Under an inert atmosphere [Co(COOCH3)2(CO)3]- undergoes elimination of dimethylcarbonate.

Abstract: In order to understand the trends of the M4 skeleton to deform in planar clusters of type M4(CO)16, extended Huckel calculations have been performed. Depending on the total electron count the shape transforms ideally from square to rhombus through the formation of one diagonal M-M bond (64 --> 62e-). The nature of the annular and transannular M-M sigma-interactions is illustrated.

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract:

Abstract: