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

Article abstract of DOI:10.1246/bcsj.53.82

Tris(acetylacetonato<2-14C>)-cobalt(III), -chromium(III), -ruthenium(III), and -rhodium(III) undergo ligand exchange in acetylacetone(Hacac) at 85-190 deg 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) above Cr(III) above Ru(III) above Rh(III), k₀/10^-5 s^-1 being 2.4 (93 deg C), 5.6 (117 deg C), 9.5 (158 deg C), and 2.4 (185 deg 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 S_N1 and the S_N2 mechanism are assigned to the exchange reactions of the Co(III) complex and the others, respectively, for the rate-determining steps.