Intramolecular charge-transfer decomposition of the µ-peroxo-bis[penta-amminecobalt(III)]complex in acidic solutions. Stabilization by protonation and the effect of chloride and sulphate

Abstract

The kinetics of the decomposition of the µ-peroxo-bis[penta-amminecobalt(III)] complex, [(NH₃)₅Co·O₂·Co(NH₃)₅]⁴⁺ [graphics omitted] 2Co²⁺+ O₂+ 10 NH₄⁺, generated by fast one-electron reduction of the µ-superoxo-complex, [(NH₃)₅·Co·O₂·Co(NH₃)₅]⁵⁺, have been studied using the stopped-flow technique at [H⁺]= 0.005–0.100M, I= 0.10M(LiClO₄). The [H⁺] dependence of first-order rate constants, k_obs=kK/([H⁺]+K), is consistent with the formation of a protonated non-reactive form. At 25 °C the rate constant for decomposition of the unprotonated (brown) form is k= 84 s^–1, and ΔH^‡= 17.5 ± 1.0 kcal mol^–1, ΔS^‡= 8.7 ± 3.7 cal K^–1 mol^–1, in excellent agreement with previous data for solutions [NH₃]= 4.3–15.0M,I= 2.0M(NH₄NO₃). The acid dissociation constant K of the protonated (red) complex (25 °C) is 0.084M, with ΔH= 6.0 ± 2.5 kcal mol^–1, ΔS= 15.2 ± 9.0 cal K^–1 mol^–1. Chloride < sulphate further stabilize the complex, and with [Cl^–] > 0.6M the effect of [H⁺] and [Cl^–] on the rate of decomposition was studied by conventional spectrophotometry, I= 2.3M(Cl^–/ClO₄). Solid samples of the brown and red complexes interconvert rapidly in solution to give identical spectra. The protonated red complex does not react with iodide during the ca. 30 min period required for decomposition. Implications regarding the structure of the protonated complex are considered.