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

Abstract

The radical anions Br₂^– and Cl₂^–, generated by flash photolysis of solutions containing the trihalide ions, oxidize manganese(II) to manganese(III). The reactions are first order in manganese(II) concentration at low concentrations but tend to zero order in manganese(II) at concentrations >10^–2M. The kinetics are interpreted in terms of the formation of manganese(II)–radical ion complexes, followed by metal-to-ligand electron transfer in the complexes [equations (i) and (ii); X = Br or Cl]. X₂^–+ Mn²⁺_aq [graphic omitted] (Mn^IIX₂⁺)(i), (Mn^IIX₂⁺)→ Mn^III+ 2X^–(ii) The reaction mechanism can be classified as inner sphere–electron-transfer controlled. At 25 °C and I= 0·25 mol I^–1, the equilibrium constants K are 39 ± 15 (Br₂^–) and 68 ± 30 l mol^–1(Cl₂^–) and the rate constants k are (2·3 ± 0·5)× 10⁵(Br₂^–) and (2·1 ±0·5)× 10⁵s^–1(Cl₂^–). The overall observed activation energies are 36 ± 4 and 34 ± 4 kJ mol^–1 for the Mn^II–Br₂^– and Mn^II–Cl₂^– reactions respectively.