An NMR study of the fluxionality of group VI metal pentacarbonyl complexes of 1,3-dithiolanes

Abstract

The complexes [M(CO) 5( SC 2H 4SC RR′)] (M = Cr, Mo or W, R = R′= H; M = Cr or W, R = R′= Me, R = H, R′= Me, Bu t or Ph) have been synthesized and characterized. Their solution stereodynamics were examined by 1H NMR spectroscopy. Pyramidal inversion of the metal-coordinated sulphur atoms occurs rapidly at ambient temperatures (Δ G‡ (298 K) = 40–44 kJ mol −1). An appreciably slower 1,3-metal—sulphur shift is also evident at temperatures where sulphur inversion is rapid on the proton NMR timescale. For the ligand complexes (R = R′ = H, Me) this shift is purely intramolecular in nature (Δ G† (298 K) values ∼ 78 kJ mol −1), in contrast to the 2-mono-substituted ligand complexes (R = H, R′ = Me, Bu t, Ph) where it is accompanied by a dissociation process.