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

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).