Reactions of µ-hydroxo-dicobalt(III) complexes. Part IX. Equilibrium and kinetic studies of the reaction of the µ-amido-µ-hydroxo-bis[tetra-amminecobalt(III)] complex with selenate ions in aqueous perchloric acid solutions, and characterisation of the µ-amido-µ-selenato-bis[tetra-amminecobalt(III)] complex

Abstract

Perchlorate and bromide salts of the µ-amido-µ-selenato-bis[tetra-amminecobalt(III)] complex have been prepared and characterised. Equilibrium and kinetic studies on the interconversion [equation (i)] of the µ-amido-µ-hydroxo-(I) and µ-amido-µ-selenato-complexes (III) at I= 2·0M(NaClO₄) are described. [graphic omitted] From spectrophotometric studies at 25 °C, the overall equilibrium constant K₁₂= 265 ± 5 I² mol^–2. For the forward reaction, the conversion of (I) into (II) is rate determining and first-order rate constants, k_f, show the dependence k_f/S^–1=k_1a[H⁺][SeO₄^2–]. At 25 °C, k_1a= 1·35 × 10^–2 I² mol^–2 s^–1, ΔH_1a^‡= 18·2 ± 0·3 kcal mol^–1 and ΔS_1a^‡=–6·3 ± 0·9 cal K^–1 mol^–1. The reverse reaction shows a dependence on [H⁺] of the form k_r/s^–1=a+b[H⁺] over the temperature range 45–60 °C, where k_r=k_–2k_–1/k₂. At 25 °C extrapolated values of a and b are 3·85 × 10^–5 s^–1 and 8·6 × 10^–6 l mol^–1 s^–1 respectively. Activation parameters are ΔH_a^‡= 16·4 ± 0·4 kcal mol^–1, ΔS_a^‡=–30·2 ± 1·4 cal K^–1 mol^–1; and ΔH_b^‡= 16·4 ± 1·6 kcal mol^–1 and ΔS_b^‡=–36·3 ± 4·6 cal K^–1 mol^–1.