Kinetics and mechanism of reduction of tris(acetylacetonato)manganese(III) by HONH(SO3)- and HON(SO3)22- in aqueous solution

Abstract

The kinetics and mechanism of reduction of tris(acetylacetonato)manganese(III) [Mn ^III (acac) ₃ ] by HONH(SO ₃ ) ^- and HON(SO ₃ ) ₂ ^2- in aqueous solution was studied as a function of pH, [SN oxide], [acac], temperature and pressure. The oxidation of [Mn ^II (acac) ₂ ] by one of the oxidation products of HON(SO ₃ ) ₂ ^2- , viz. ON(SO ₃ ) ₂ ^2- , was studied as a function of pH, [ON(SO ₃ ) ₂ ^2- ], [Mn ^II ], [Mn ^III ] and pressure. The activation parameters for the reduction by HONH(SO ₃ ) ^- at pH 5.2, viz ΔH ^‡ = 63 ± 2 kJ mol ^-1 , ΔS ^‡ = 8 ± 6 J K ^-1 mol ^-1 and ΔV ^‡ = -5.8 ± 0.5 cm ³ mol ^-1 , and for the oxidation by ON(SO ₃ ) ₂ ^2- , viz. ΔV ^‡ = - 13.4 ± 0.7 cm ³ mol ^-1 , are discussed in terms of the nature of the electron-transfer mechanism. The kinetic data confirm the inner-sphere nature of the reactions when SN oxides react with manganese(III) complexes in general. Both the reduction and oxidation reactions are substitution-controlled redox processes. Oxidation products are generated during the reduction of the trivalent metal-ion species, which participate in the reoxidation of the metal to its trivalent state. The results are discussed with reference to data available for related reactions.