KINETICS AND MECHANISM OF REDUCTION OF TRIS(ACETYLACETONATO)-MANGANESE(III) BY HONH(SO3)(-) AND HON(SO3)(2)(2-) IN AQUEOUS-SOLUTION

The kinetics and mechanism of reduction of tris(acetylacetonato)mangan ese(III) [Mn-III(acac)(3)] by HONH(SO3)(-) and HON(SO3)(2)(2-) in aque ous solution was studied as a function of pH, [SN oxide], [acac], temp erature and pressure. The oxidation of [Mn-II(acac)(2)] by one of the oxidation products of HON(SO3)(2)(2-), viz. ON(SO3)(2)(2-), was studie d as a function of pH, [ON(SO3)(2)(2-)], [Mn-II], [Mn-III] and pressur e. The activation parameters for the reduction by HONH(SO3)(-) at pH 5 .2, viz Delta H-double dagger = 63 +/- 2 kJ mol(-1), Delta S-double da gger = 8 +/- 6 J K-1 mol(-1) and Delta V-double dagger = -5.8 +/- 0.5 cm(3) mol(-1), and for the oxidation by ON(SO3)(2)(2-), viz. Delta V-d ouble dagger = -13.4 +/- 0.7 cm(3) mol(-1), are discussed in terms of the nature of the electron-transfer mechanism. The kinetic data confir m the inner-sphere nature of the reactions when SN oxides react with m anganese(III) complexes in general. Both the reduction and oxidation r eactions are substitution-controlled redox processes. Oxidation produc ts are generated during the reduction of the trivalent metal-ion speci es, which participate in the reoxidation of the metal to its trivalent state. The results are discussed with reference to data available for related reactions.