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

Daniel L DuBois, W. K. Miller, M. Rakowski DuBois

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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 [CH3C5H4MoIV(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-MoIIISC2H 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.

Original languageEnglish
Pages (from-to)3429-3436
Number of pages8
JournalJournal of the American Chemical Society
Issue number12
Publication statusPublished - 1981


ASJC Scopus subject areas

  • Chemistry(all)

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