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

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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.

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