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

• Published in: Inorganica Chimica Acta 2000-04, Vol.300-302, p.668-674
• Main Authors: Farrar, David H., Poë, Anthony J., Zheng, Ying
• Format: Article
• Language: English
• Subjects: Kinetic studies; Polynuclear metal carbonyls; Reversible adduct formation; Stopped-flow kinetics; Thermodynamic studies
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(99)00583-6

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.