Kinetic studies and a molecular orbital interpretation of reactions at bridging sulfur ligands in dimeric molybdenum complexes

Kinetic studies of previously reported exchange reactions at the bridging sulfur atoms in cyclopentadienylmolybdenum dimers have been carried out. The kinetics of the reaction of [CH₃C₅H₄Mo^IV(S)SH]₂ (I) with benzyl isocyanide which results in the formation of H₂ and [CH₃C₅H₄MoS₂CNCH₂C ₆H₅]₂ (II) have been studied by the method of initial rates. The reaction shows a first-order dependence on the molybdenum dimer I and a first-order dependence on isocyanide with k = (7.8 ± 1.0) × 10^-4 L mol^-1 s^-1 at 31.5°C. Activation parameters have been derived from rate studies of the reaction over a temperature range of 0-55°C. The ΔH^‡ for the reaction is 7.9 kcal/mol and ΔS^‡ = -38 cal K^-1 mol^-1. The reaction of [CH₃C₅H₄-Mo^IIISC₂H ₄S]₂ (III) with benzyl isocyanide results in the formation of the same dithiocarbonimidate complex, II, and ethylene. Studies of the initial rates of the latter reaction reveal a first-order dependence on the molybdenum complex III, but a zero-order dependence on isocyanide with k = (1.0 ± 0.1) × 10^-4 s^-1 at 31.5°C. Studies over the temperature range 20-55°C established the following activation parameters: ΔH^‡ = 24.3 kcal/mol and ΔS^‡ = 11 cal K^-1 mol^-1. The work suggests that the Mo(IV) dimer reacts by an associative mechanism, while the reaction of the Mo(III) derivative proceeds by a dissociative pathway. Extended Hückel calculations for a series of molybdenum dimers with bridging sulfur ligands have been completed. The different mechanisms of reaction for the Mo(IV) and -(III) dimers are discussed in terms of the molecular orbitals involved. An 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 the Mo₂S₄ core changes as the metal oxidation state varies from V to IV or III, and this structural difference is interpreted in terms of the relative energies of the frontier molecular orbitals.