Kinetics and mechanism of inversion at sulphur and of ligand exchange in some sulphide complexes of chromium and tungsten pentacarbonyl. A case of diastereomeric fluxionality

Abstract

The chromium and tungsten pentacarbonyl complexes of 2,5-dihydrothiophene and β-phenethylmethyl sulphide have been prepared. The β-phenethylmethyl sulphide complexes can be shown by 13C and 1H NMR spectroscopy at 100°C to consist of two diastereomers, consistent with nonplanarity of the sulphur atom. For the case of the β-phenethylmethyl sulphide chromium pentacarbonyl Δ H‡ 10.0 kcal/mol and Δ S‡ 2.4 e.u. as derived from line-broadening experiments. Inversion barriers in other complexes lie in the same range. Mechanisms for the inversion process are considered. Ligand exchange takes place at higher temperatures than those required for line coalescence for the inversion process. The kinetics of this process were investigated for the exchange between the 2,5-dihydrothiophenechromium pentacarbonyl and β-phenethylmethyl sulphide. The kinetic data are interpreted in terms of a rate-determining dissociation of the sulphide ligand from the chromium atom.