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, Keq, for the equalibration in (i) can be expressed as in equation (ii), where K1 and K2 are for hydroxo-bridge cleavge, and Kb for the back reaction (product keq=k1[H+][Br–]+k2[Br–]+kb(ii) of a rate constant and the equilibrium constant kB) 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 kAkB=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).