Kinetic and thermodynamic studies of reversible adduct formation between Ru5C(CO)15 and acetonitrile

Abstract

Stopped-flow kinetic measurements have been made on the reversible addition of NCMe to Ru5C(CO)15 in 1,2-dichloroethane to form the adduct in which a Ru-Ru bond in the cluster has been broken and replaced by a Ru-NCMe bond. The measurements lead to the kinetic parameters kf (25°C)=24.3±1.4 M−1 s−1, ΔHf≠=31.7±3.5 kJ mol−1 and ΔSf≠=−91±12 J K−1 mol−1; kr (25°C)=296±18 s−1, ΔHr≠=51.0±1.3 kJ mol−1 and ΔSr≠=−47±4 J K−1 mol−1. Addition of NCMe requires an exceptionally low activation enthalpy and suggests a very early transition state, perhaps with attack at the carbon atom of a coordinated CO ligand. Loss of NCMe from the adduct also suggests a late transition state for the reverse process with a great deal of Ru-Ru bond making and a consequently pronounced negative entropy of activation. These data, when combined with UV–Vis measurements on reactions at equilibrium, lead to the thermodynamic parameters K (25°C)=12.6 M−1, ΔH°=−17.6±1.2 kJ mol−1 and ΔS°=−38±4 J K−1 mol−1. These novel parameters provide a quantitative measure of the release of strain within the Ru5C cluster when it is opened up by reaction with the rather weak ligand NCMe.