Iron−Manganese Redox Processes and Synergism in the Mechanism for Manganese-Catalyzed Autoxidation of Hydrogen Sulfite

The mechanism for manganese-catalyzed aqueous autoxidation of hydrogen sulfite at pH 2.4 has been revised on the basis of previous comprehensive kinetic studies and thermodynamic data for iron−manganese redox processes and manganese(II) and -(III) protolysis equilibria. The catalytically active manganese species is concluded to be an oxo- (or hydroxo-) bridged mixed-valence complex of composition (OH)MnIIIOMnII(aq) with a formation constant β‘ of (3 ± 1) × 104 M-1 from kinetics or ca. 7 × 104 M-1 from thermodynamics. It is formed via rapid reaction between Mn(H2O)6 2+ and hydrolyzed manganese(III) aqua hydroxo complexes, and it initiates the chain reaction via formation of a precursor complex with HSO3 -, within which fast bridged electron transfer from S(IV) to Mn(III) takes place, resulting in formation of chain propagating sulfite radicals, SO3 •-. The very high acidity of Mn3+(aq), indicating a strong bond MnIII−OH2 in hydrolyzed manganese(III), makes an attack by HSO3 - on substitution labile Mn(II) in the bridged complex more favorable than one directly on manganese(III). The synergistic effect observed in systems containing iron as well as manganese and the chain initiation by trace concentrations of iron(III) of ca. 5 × 10-8 M can also be rationalized in terms of formation of this bridged mixed-valence dimanganese(II,III) complex. The presence of iron(III) in a Mn(II)/HSO3 - system results in rapid establishment of an iron−manganese redox equilibrium, increasing the concentration of manganese(III) and of the catalytically active bridged complex. The bridged complex oxidizes HSO3 - several orders of magnitude faster than does iron(III) itself. Comparison with some previous studies shows that the different experimental rate laws reported do not necessarily indicate different reaction mechanisms. Instead, they can be rationalized in terms of different rate-determining steps within the same complex chain reaction mechanism, depending on the experimental conditions used.