Reactions of µ-hydroxo-dicobalt(III) complexes. Part XII. Equilibrium and kinetic studies on hydroxo-bridge cleavage reactions of two triply-bridged dicobalt(III) complexes in aqueous perchloric acid solutions

Abstract

The equilibration kinetics of the first hydroxo-bridge cleavage of the triply-bridged complexes tri-µ-hydroxo-bis[triamminecobalt(III)], [(NH₃)₃Co·µ(OH,OH,OH)·Co(NH₃)₃]³⁺, (I), and µ-amido-di-µ-hydroxo-bis[tri-amminecobalt(III)], [(NH₃)₃Co·µ(NH₂,OH,OH)·Co(NH₃)₃]³⁺, (II), have been studied, I= 1·0M(LiClO₄). Within the range [H⁺]= 0·01–0·5M first-order rate constants, k_obs, for the equilibration of (I) fit the dependence (i)k_obs=k₁K_P[H⁺]//_{1 +KP[H⁺]}+k_–1(i) where K_P, k₁, and k_–1 are for the reaction sequence (ii) and (iii). [graphic omitted] (ii), [graphic omitted] (iii) At 25 °C K_P= 1·51 (±0·18) l mol^–1, k₁= 1·86 (±0·15)× 10^–1 s^–1, and k_–1= 6·62 (±0·36)× 10^–3 s^–1. For (II) the dependence of k_obs′ on [H⁺] is of the simpler form (iv), k_obs′=k₂K_P′[H⁺]+k_–2(iv) and at 25 °C K_P′ <0·05 l mol^–1, k₂K_P′= 1·01 (±0·02) l mol^–1 s^–1, and k_–2= l·96 (±0·4)× 10^–2 s^–1. Activation parameters for k₁, k_–1, k₂K_P′, and k_–2 have been determined. Equilibrium data have also been evaluated from spectrophotometric measurements.