Reactions of µ-hydroxo-dicobalt(III) complexes. Part XI. Kinetic studies on the reaction of bromide with the µ-amido-µ-hydroxo-bis[ tetra-amminecobalt(III)] complex

Abstract

The kinetics of hydroxo-bridge cleavage of the µ-amido-µ-hydroxo-bis[tetra-amminecobalt(III)] complex (I) by bromide in aqueous perchioric acid have been investigated spectrophotometrically with [Br^–]= 05–2·0M, [H+]= 0·5–2·0M, and I= 2·0M(NaClO₄). The product in the rate-determining step is the aquo-bromo-complex, (II), which is in rapid equilibrium with the µ-amido-µ-bromo-complex (III), equation (i). The bromo-complex [graphic omitted] (III) and not (II) is predominant. Pseudo-frist-order rate constant, K_eq, for the equalibration in (i) can be expressed as in equation (ii), where K₁ and K₂ are for hydroxo-bridge cleavge, and K_b for the back reaction (product k_eq=k₁[H⁺][Br^–]+k₂[Br^–]+k_b(ii) of a rate constant and the equilibrium constant k_B) is as determined previously. At 25 °C k₁=(4·33 ± 0·17)× 10^–4l² mol^–2 s^–1, k₂=(1·4 ± 0·2)× 10^–4l mol^–1 s^–1, and k_Ak_B=ca.1·1 l² mol^–2. From the temperature dependence ΔH₁^‡= 17·3 ± 0·8 kcal mol^–1 s^–1,ΔS₁^‡=–16·2 ± 2·6 cal mol^–1 k^–1,ΔH₂^‡= 25·7 ± 1·8 kcl mol^–1[graphic omitted] and ΔS₂^‡= 10·8 ± 5·7 cal mol^–1 k^–1. Reaction (iii) is effective at [BR^–] > 1·7M, where (II) may be involved as an intermediate. At 25 °C k₃=ca. 0·03 l mol^–1, and k_–3 > 2 × 10^–2 s^–1(at 1·5 °C),I= 2·0M(NaClO₄).