Kinetics of oxidation of transition-metal ions by halogen radical anions. Part I. The oxidation of iron(II) by dibromide and dichloride ions generated by flash photolysis

Abstract

The radical anions Br₂^– and Cl₂^–, generated by flash photolysis of the corresponding halide solutions or of the iron(III) complexes FeBr²⁺ and FeCl²⁺, oxidize iron(II) to iron(III). The product of the reaction between the ions Br₂^– and Fe²⁺ is the FeBr²⁺ complex. The reaction proceeds by an inner-sphere substitution-controlled path with a rate constant of (3·6 ± 0·4)×10⁶ l mol^–1 s^–1 at 25 °C (ΔH^‡= 25·2 ± 2 kJ mol^–1; ΔS^‡=–42 ± 12 J K^–1 mol^–1). The reaction between the ions Cl₂^– and Fe²⁺ proceeds by two paths at 25 °C, an inner-sphere substitution-controlled path with a rate constant of (4·0 ± 0·6)× 10⁶ l mol^–1 s^–1(ΔH^‡= 31·5 ± 4 kJ mol^–1; ΔS^‡=–21 ± 15 J K^–1 mol^–1), and an outer-sphere path with a rate constant of (1·0 ± 0·2)× 10⁷ l mol^–1s^–1(ΔH^‡= 22·7 ± 4 kJ mol^–1; ΔS^‡=–42 ± 15 J K^–1 mol^–1).