Reactions of µ-hydroxo-dicobalt(III) complexes. Part VIII. Kinetic studies of the bridge cleavage of the µ-hydroxo-bis[penta-amminecobalt(III)] complex

Abstract

The kinetics of bridge cleavage of the µ-hydroxo-bis[penta-amminecobalt(III)] complex, (NH₃)₅Co·OH·Co(NH₃)₅⁵⁺[equation (i)] have been studied in aqueous perchloric acid solutions, I= 2·0M(LiClO₄), at 15–30 °C. (NH₃)₅Co·OH·Co(NH₃)₅⁵⁺+ H⁺+ H₂O → 2Co(NH₃)₅H₂O³⁺(i) For the range of hydrogen-ion concentration investigated (10^–4–2·0M), first-order rate constants, k_obs, show the dependdence (ii) on [H⁺]. k_obs=k₁[H⁺]+k₂(ii) At 25 ° C: k₁=(5·37 ± 0·07)× 10^–3 l mol^–1 s^–1, ΔH₁^‡= 12·0 ± 0·8 kcal mol^–1, ΔS₁^‡=–28·7 ± 2·7 cal K^–1 mol^–1; and k₂=(7·58 ± 0·10)× 10^–3 s^–1, ΔH₂^‡= 20·1 ± 0·70 kcal mol^–1, ΔS₂^‡=–1·0 ± 2·2 cal K^–1 mol^–1. In the presence of thiocyanate ions (0·06–0·80M), K_obs, can be expressed as in (iii), where a and bare ca. 15% smaller than k₁ and k₂ due to ion-pairing. k_obs=a[H⁺]+b+c[H⁺][NCS^–](iii) At 25 °C and I= 2·0M(LiClO₄) : c= 0·137 ± 0·003 I² mol^–2 s^–1, ΔH_c^‡= 10·9 ± 0·6 kcal mol^–1, and ΔS_c^‡=–25·8 ± 2·2 cal K^–1 mol^–1. The thiocyanate-dependent path yields equal amounts of the complexes Co(NH₃)₅NCS²⁺ and Co(NH₃)₅H₂O³⁺ as primary products.