Kinetics of the monosubstitution of chloride by thiocyanate, bromide, and iodide in tetrachloroaurate(III) ions in aqueous solution: the spectroscopic detection of reaction intermediates, and catalysis by chloride ions

Abstract

Kinetic and spectroscopic evidence is presented for the occurrence of intermediates in monosubstitution of [AuCl₄]^– ions by thiocyanate and bromide ions in aqueous solution. The intermediate(s) isformed in a pre-equilibrium and then undergoes unimolecular decomposition to the products. The rates of both stages of the reaction have been studied using stopped-flow spectrophotometry; both stages exhibit a mild catalysis by added chloride ions. The monosubstitution of [AuCl₄]^– by iodide ions displays no evidence for an intermediate but the overall reaction is again catalysed by chloride ions, here powerfully. Possible reaction mechanisms are discussed in the light of previous conclusions about the mechanisms of similar square-planar substitutions.