KINETICS AND MECHANISM OF REDUCTION OF GOLD(III) COMPLEXES BY DIMETHYL SULFIDE

The reactions between trans-[Au(CN)(2)X-2](-) (X = Cl or Br) and Me2S have been studied by conventional and high-pressure stopped-flow spect rophotometry in acidic aqueous solution containing 10% (w/w) methanol. The overall stoichiometry Au-III:Me2S is 1:(1.0 +/- 0.1) in agreement with the reaction: trans-[Au(CN)(2)X-2](-) + Me2S + H2O --> [Au(CN)(2 )](-) + Me2SO + 2H(+) + 2X(-). Initial rapid substitution processes re sult in the formation of a pre-equilibrium between transient gold(III) complexes, which are reduced to [Au(CN)(2)](-) in a subsequent slower redox process. Complexes trans-[Au(CN)(2)X(Me2S)] with an asymmetric electron distribution along the X-Au-S axis are reduced rapidly via an intermolecular process, in which Me2S attacks a co-ordinated halide. The complex trans-[Au(CN)(2)(Me2S)(2)](+), on the other hand, undergoe s slow reduction to gold(I) involving a water molecule. The rapid hali de-mediated oxidation of thioethers implies that reduction of metal io ns in biological systems by such moieties should be favoured in extrac ellular environments, where the chloride concentrations are high.