Kinetic Studies and Molecular Orbital Interpretation of Reactions at Bridging Sulfur Ligands in Dimeric Molybdenium Complexes

Article abstract of DOI:10.1021/ja00402a029

Kinetic studies of previously reported exchange reactions at the bridging sulfur atoms in cyclopentadienylmolybdenium dimers have been carried out.The kinetics of the reacton of IV(S)SH>2 (I) with benzyl isosyanide which results in the formation of H2 and 2 (II) have been studied by the method of initial rates.The reaction shows the first-order dependence on the molybdenium dimer I and the first-order dependence on isocyanide with k=(7.8+/-1.0)*10^-4 L mol^-1 s^-1 at 31.5 deg C.Activation parameters have been derived from rate studies of the reaction over a temperature range of 0-55 deg C.The ΔH^<*> for the reaction is 7.9 kcal/mol and ΔS^<*> = -38 cal K^-1 mol^-1.The reaction of the IIISC2H4S>2(III) with benzyl isocyanide results in the formation of the same dithiocarbonimidiate complex, II, and ethylene.Studies of the initial rates of the latter reaction reveal a first-order dependence on molybdenium complex III, but a zero-order dependence on isocyanide with k=(1.0+/-0.1)*10^-4 s^-1 at 31.5 deg C.Studies over the temperature range 20-55 deg C established the following activation parameters: ΔH^<*>0 24.3 kcal/mol and ΔS^<*> 0 11 cal K^-1 mol ^-1.The work suggests that the Mo(IV) dimer reacts by associative mechanism, while the reaction of Mo(III) derivative proceeds by dissociative pathway.Extended Huskel calculations for a series of molybdenium dimers with bridging sulfur ligands have been completed.The different mechanisms of the reaction for the Mo(IV) and-(III) dimers are discussed in terms of molecular orbials involved.The analogy is made between the molecular orbitals of the sulfur ligands in these dimers and those of 16- and 18-electron metal centers in organometallic complexes.The observed structure of Mo2S4 core changes as a melal oxydation state varies fom V to IV or III, and this structural dependence is interpreted in terms of the relative energies of the frontier molecular orbitals.