The ligand-substitution reactions of aquahydroxocobinamide proceed through a dissociative interchange mechanism

Abstract

The dependence of the second-order rate constants for replacement of H₂O in aquahydroxocobinamide by azide at 25.0 °C, ionic strength l= 1.0 mol dm^–3(KCl) in the range pH 9–12 showed that dihydroxocobinamide is inert to substitution. The kinetics of substitution of bound H₂O in aquahydroxocobinamide by L = cyanide, azide, pyridine, N-methylimidazole or 3-aminopropan-1-ol was investigated as a function of ligand concentration and temperature by stopped-flow spectrophotometry at pH 12.0 and a constant l of 2.0 mol dm^–3(except for 3-aminopropan-1-ol where l= 1.0 mol dm^–3 because of the limited solubility of the ligand). The observed pseudo-first-order rate constants (corrected, where appropriate, for protonation of the N-donor atom of L, and the presence of inert dihydroxocobinamide) showed the onset of saturation with ligand concentration for all ligands, with the exception of 3-aminopropan-1-ol. The saturation effect proves that the reaction proceeds through a dissociative activation pathway. Furthermore, the observation that the saturation rate constant, k_sat(and its activation parameters ΔH^‡ and ΔS^‡), depends on the identity of L indicates that incoming L participates in the transition state. This allows the mechanism of the reaction to be identified as a dissociative interchange.