Kinetics and mechanism of the base hydrolysis of the cis-chlorobis(ethylenediamine)(imidazole)cobalt(III) cation

Abstract

Base hydrolysis of cis-[CoCl(en)₂(imH)]²⁺(en = ethylenediamine, imH = imidazole) has been studied over the range pH 8.26–9.74 at l= 0.1 mol dm^–3(Na[ClO₄]) and 25 °C. In this pH range the reaction is first order in the hydroxide-ion concentration (k_OH 29 ± 3 dm³ mol^–1 s^–1). At higher pH (9.89–12.89) the reaction displays less than a first-order dependence on the [OH^–] due to conversion into the conjugate base cis-[CoCl(en)₂(im)]⁺(im^–= deprotonated imidazole). The pK_a for this ionisation is 10.5 at l= 0.1 mol dm^–3. At still higher pH the values of k_obs. do not become constant, but increase as the pH is increased, consistent with the formation of a second conjugate base by deprotonation of an ethylenediamine ligand. The activation parameters for base hydrolysis at low pH are ΔH^‡= 91.2 kJ mol^–1 and ΔS^‡(298 K)= 89 J K^–1 mol^–1. The substantial positive ΔS^‡ is fully consistent with an S_N1(CB) mechanism.