Kinetics of oxygen-atom transfer reactions involving molybdenum dithiolene complexes

Oxo transfer reactions from [MoO2(mnt)(2)](2-) [mnt(2-) = 2,3-disulfanylmaleonitrile dianion (1,2-dicyanoethylene-dithiolate)] to the abiological oxygen acceptors PPh3-xEtx (x = 0-3) have been studied spectrophotometrically in acetonitrile. The pseudo-first-order rate constants depend linearly on the concentration of the oxo accepters. The sequence of reactivity is PEt3 > PEt2Ph > PEtPh2 > PPh3, reflecting the basicities of the phosphines, while the steric influence of the substituents on the phosphorus atom appears to be of minor importance. The activation entropies for the reaction with PEt3 and PPh3 are negative. These observations are in agreement with the proposal that in the intermediate the entering phosphine binds to the molybdenum complex through the oxygen ligand. Oxidation of HSO3- by [MoO2(mnt)(2)](2-) has also been studied in acetonitrile and in acetonitrile-water mixtures. The reaction exhibits Michaelis-Menten behaviour, In CH3CN-H2O, the rate of the reaction increases as the water concentration decreases; the reacting species appears to be a product from the reaction of the molybdenum starting complex with water.