Kinetics and mechanism of aquation of cis (ammine)bis(ethylenediamine) (substituted salicylato)cobalt(III) complexes

Abstract

The kinetics of aquation of cis (ammine)bis(ethylenediamine) (substituted salicylato)cobalt(III) complexes, [(en)₂(NH₃)CoCO₂C₆H₃(X)OH]²⁺ (X = H, 5-SO₃^-, 5-Br, 5-NO₂, 3-NO₂), has been investigated in aqueous perchlorate medium of I = 2.0 M and at 55-70°. The observed rate constant for aquation over the acidity range [H⁺] = 0.1-2.0 M is given by k_obs = k₀ + k₁[H⁺] where k₀ and k₁ are the rate constants associated with the spontaneous and acid catalysed paths respectively. Attempt to determine the protonation constant of the complexes was unsuccessful. The rate and activation parameters for the k₀ and k₁ paths have been determined. k₀ increases with increasing electron withdrawing effect of the substituents, but this decreases k₁. The substituent effect on both the paths is, however, small. The activation parameters for both paths obey the isokinetic relationship. Both SO₄^- and HSO₄^- catalyse the aquation process. The stability and reactivity of the sulphate ion-pairs have been assessed. The log k₀ vs pK_eq plot, where K_eq is the formation constant of the salicylato complexes, is linear with unit slope. The results support rate limiting by Co---O bond breaking in the k₀ and k₁ paths.