Complex formation between iron(II) and inorganic anions. Part II. The effect of oxyanions on the reaction of iron(II) with hydrogen peroxide

Abstract

The increase in the bimolecular rate constant k₀ for the reaction of iron(II) with hydrogen peroxide found with added halide ions is also observed with added sulphate, selenate, trimetaphosphate, and triphosphate ions. k₀ increases to a limiting value which is different for each anion. Following the interpretation of the results with halide ions, 1:1 complexes of anions with Fe^II ions are found. Heats ΔH and entropies ΔS of formation of FeSO₄, FeSeO₄, FeHSO₄ ⁺, FeHSeO₄ ⁺, FeP₃O₉ ^–, and FeHP₃O₉ are compared with ΔH and ΔS for the halogenoaquoiron(II) complexes and the hydroxoaquoiron(ii) ion. ΔH and ΔS are related to changes in the solvent structure induced by transferring the anion from bulk solvent to the Fe^II ion. Comparison is also made with ΔH and ΔS for other cation–ligand systems. Energies E and entropies ΔS* of activation for the reactions of these oxyanion aquoiron(II) complexes with H₂O₂ are compared with E and ΔS* for the reactions of halogenoaquoiron(II) complexes with H₂O₂.