Kinetics of Ligand Exchange between Tris(acetylacetonato[2-14C])cobalt(III) and Acetylacetone in Acetonitrile, Evidence for SN1 Mechanism

Abstract

Abstract Tris(acetylacetonato[2-14C]) cobalt (III) undergoes ligand isotopic exchange with acetylacetone (Hacac) in acetonitrile at 85–100 °C without decomposition and solvolysis. The exchange rate is proportional to the complex concentration (0.002–0.007 M, 1 M=1 mol dm−3), the first-order rate constant ko being independent of [Hacac] (0.001–0.97 M). ko=1.6×10−5 s−1 at 93.2 °C. The activation enthalpy and entropy are 38±4 kcal mol−1 (1 cal= 4.18J) and 23±8 cal K−1 mol−1, respectively. Trichloroacetic acid (<0.02 M) gives acid catalysis. The ratedetermining step should be the SN1 Co–O cleavage in a chelate ring to give a five coordinate intermediate.