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A Comparison of the Influences of Alkoxide and Thiolate Ligands on the Electronic Structure and Reactivity of Molybdenum(3+) and Tungsten(3+) Complexes. Preparation and Structures of M2(OtBu)2(StBu)4, [Mo(StBu)3((NO)]2, and W(StBu)3(NO)(py)

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journal contribution
posted on 07.09.2001, 00:00 by Malcolm H. Chisholm, Ernest R. Davidson, John C. Huffman, Kristine B. Quinlan
M2(OtBu)6 compounds (M = Mo, W) react in hydrocarbon solvents with an excess of tBuSH to give M2(OtBu)2(StBu)4, red, air- and temperature-sensitive compounds. 1H NMR studies reveal the equilibrium M2(OtBu)6 + 4tBuSH ⇌ M2(OtBu)2(StBu)4 + 4tBuOH proceeds to the right slowly at 22 °C. The intermediates M2(OtBu)4(StBu)2, M2(OtBu)3(StBu)3, and M2(OtBu)5(StBu) have been detected. The equilibrium constants show the M−OtBu bonds to be enthalpically favored over the M−StBu bonds. In contrast to the M2(OtBu)6 compounds, M2(OtBu)2(StBu)4 compounds are inert with respect to the addition of CO, CO2, ethyne, tBuC⋮CH, MeC⋮N, and PhC⋮N. Addition of an excess of tBuSH to a hydrocarbon solution of W2(OtBu)6(μ-CO) leads to the rapid expulsion of CO and subsequent formation of W2(OtBu)2(StBu)4. Addition of an excess of tBuSH to hydrocarbon solutions of [Mo(OtBu)3(NO)]2 and W(OtBu)3(NO)(py) gives the structurally related compounds [Mo(StBu)3(NO)]2 and W(StBu)3(NO)(py), with linear M−N−O moieties and five-coordinate metal atoms. The values of ν(NO) are higher in the related thiolate compounds than in their alkoxide counterparts. The bonding in the model compounds M2(EH)6, M2(OH)2(EH)4, (HE)3M⋮CMe, and W(EH)3(NO)(NH3) and the fragments M(EH)3, where M = Mo or W and E = O or S, has been examined by DFT B3LYP calculations employing various basis sets including polarization functions for O and S and two different core potentials, LANL2 and relativistic CEP. BLYP calculations were done with ZORA relativistic terms using ADF 2000. The calculations, irrespective of the method used, indicate that the M−O bonds are more ionic than the M−S bonds and that E pπ to M dπ bonding is more important for E = O. The latter raises the M−M π orbital energies by ca. 1 eV for M2(OH)6 relative to M2(SH)6. For M(EH)3 fragments, the metal dxz,dyz orbitals are destabilized by OH pπ bonding, and in W(EH)3(NO)(NH3) the O pπ to M dπ donation enhances W dπ to NO π* back-bonding. Estimates of the bond strengths for the M⋮M in M2(EH)6 compounds and M⋮C in (EH)3M⋮CMe have been obtained. The stronger π donation of the alkoxide ligands is proposed to enhance back-bonding to the π* orbitals of alkynes and nitriles and facilitate their reductive cleavage, a reaction that is not observed for their thiolate counterpart.