The trans effect in gold(III) complexes. Kinetics of substitution reactions of the trichloro(phenyl)aurate(III) anion in methanol

Abstract

The [Au(C₆H₅)Cl₃]^– ion undergoes reversible solvolysis of the chloride trans to C₆H₅^– in methanol solution. The kinetics of the approach to equilibrium on adding excess of LiCl have been followed by stopped-flow spectrophotometry at temperatures below 0 °C and the rate and equilibrium constants determined. In the absence of added Cl^– the complex exists mainly as trans-[Au(C₆H₅)(MeOH)Cl₂]. The kinetics of the replacement of MeOH by a variety of nucleophiles have been studied. The reactivity follows the sequence NO₂^– < Cl^– < Br^– < N₃^–≈ I^– < NCS^– < thiourea but the nucleophilic discrimination is considerably less than that observed in other gold(III) complexes. The reaction with iodide leads to reduction of the substrate and the formation of I₃^– but the rate-determining step is shown to be a nucleophilic substitution. The kinetics of the reaction [Au(C₆H₅)Cl₃]^–+ nu^–→trans-[Au(C₆H₅)Cl₂(nu)]^x–+ Cl^– have been followed in methanol solution by working in the presence of excess of LiCl. Neutral nucleophiles enter by the usual direct associative pathway, the nucleophilicity increasing along the sequence pyridine < Et₂S < thiourea. Nucleophilic discrimination is low. Anionic reagents do not react directly with the anionic [Au(C₆H₅)Cl₃]^– complex but enter by displacing methanol from the solvento species. In this respect [Au(C₆H₅)Cl₃]^– resembles [Pt(L)Cl₃]^–, where L is a neutral ligand with a moderate or high σ-donor trans effect.