Kinetics; mechanism of base hydrolysis of cisα-oxalato(amine)(trien)cobalt(III); salicylato(amine)(trien)cobalt(III)cations

Abstract

Oxalato(amine)(trien)cobalt(III) and salicylato(amine)(trien)cobalt(III) perchlorates have been synthesized and tentatively assigned a cis α configuration. The dissociation constants of the complexes have been determined. The base hydrolysis of the complexes have been investigated at 30, 35, 38.6°C and I = 1.0 mol dm^-3. The rate laws for the oxalato and salicylato complexes are -dln[Complex]_T/dt = k₂[OH^-] and -dln[Complex]T = k₁ + k₂[OH^-] respectively. For the oxalato complex, k₂(30°C) = (2.95 ±0.05) × 10^-2 dm³ mol^-1 s^-1, ΔH^□ = (109 ±4) KJ mol^-1, ΔS^□ = (85 ±13) JK^-1 mol^-1 and for the salicylato complex, k1(30°C) = (1.58 ±0.28) × 10^-4s^-1, ΔH^□ = (166 ±7) KJ mol^-1, ΔS^□ = (229 ±21) JK^-1 mol^-1 and k₂(30°C) = (3.56 ±0.10) × 10^-3 dm3 mol^-1 s^-1, ΔH^□ = (101 ±6) KJ mol^-1, ΔS^□ = (42 ±20) JK^-1 mol^-1. The base hydrolysis reactions of the complexes were followed in presence of imidazole and ethanolamine, in the presence of added anions and also in D₂O medium. The results are discussed in terms of d SN₁CB mechanism involving rate limiting Co□O bond fission for both k₁ and k₂ paths. However, the possibility of C□O bond cleavage in the base hydrolysis of the oxalato complex is not ruled out.