Kinetics and Mechanisms of Ligand Exchange Reactions of Tris(acetylacetonato)-chromium(III), -cobalt(III), -ruthenium(III), and -rhodium(III) in Acetylacetone

Abstract

Tris(acetylacetonato[2-14C])-cobalt(III), -chromium(III), -ruthenium(III), and -rhodium(III) undergo ligand exchange in acetylacetone (Hacac) at 85–190 °C without decomposition of the complexes. The exchange rate is proportional to the complex concentration, and the first-order rate constant k0 decreases in the sequence Co(III)>Cr(III)>Ru(III)>Rh(III), k0⁄10−5 s−1 being 2.4 (93 °C), 5.6 (117 °C), 9.5 (158 °C), and 2.4 (185 °C), respectively. The activation enthalpies and entropies and deuterium isotope effect on k0 are significantly different between the Co(III) and the Cr(III), Ru(III) and Rh(III) complexes. An intermediate involving an one-ended acetylacetonate and a solvent molecule(Hacac) is concluded to be formed in the rate-determining step. The SN1 and the SN2 mechanism are assigned to the exchange reactions of the Co(III) complex and the others, respectively, for the rate-determining steps.