Manganese(II) Oxidation and Mn(IV) Reduction in the Environment—Two One-Electron Transfer Steps Versus a Single Two-Electron Step

Abstract

The chemistry of electron transfer processes are reviewed using a knowledge of orbital properties and available experimental data. Fe(H2O)6 2+ and Mn(H2O)6 2+ oxidation with O2 are discussed and compared. Mn(H2O)6 2+ oxidation by O2 occurs via an inner sphere process after complexation with inorganic (e.g.; OH−, increase pH) or organic ligands that replace water; whereas Fe(H2O)6 2+ at circumneutral pH occurs via an outer sphere mechanism. An outer sphere electron transfer process is symmetry forbidden for Mn(H2O)6 2+ based on analysis of the frontier molecular orbitals of the reactants. At higher pH, an inner-sphere process is also available for Fe(H2O)6 2+ oxidation as hydroxide and organic ligands replace water and bind Fe(II). The bonding of O2 to Fe(H2O)6 2+ in the precursor complex results in faster electron transfer for the inner sphere process than occurs in the outer-sphere process which occurs at lower pH. The bonding between the reactants in an inner sphere process is likely “end on” bonding for O2 to the metal with a bent M-O-O bond angle. Side-on bonding for O2 to the metal is possible and could lead to two-electron transfers from Mn(II) compounds but requires stabilization of the Mn-O2 bonding with organic ligands such as porphyrins. This would occur as an oxidative addition type reaction where the Mn(II) would give up two electrons to two different orbitals of O2 and increase its local coordination environment. For two-electron transfers during Mn(II) compound oxidation, multinuclear Mn complexes are required. One-electron transfers are more likely to occur during the oxidation of Mn(H2O)6 2+ by O2 and the reduction of MnO2 than two-electron transfers. Both soluble and solid phase Mn(III) species form as intermediates or stable species. From a microbiological viewpoint, Mn(III) compounds are ideal reagents as Mn(III) can act as an electron acceptor forming soluble Mn(H2O)6 2+ or as an electron donor forming insoluble MnO2. One-electron transfers are predicted based on the different spatial characteristics of the d z2 and d x2−y2 orbitals. The d z2 orbital has electron density on all three Cartesian coordinate axes (primarily the z axis) but the d x 2−y2 orbital has electron density only in the xy plane. Adding or losing two electrons simultaneously is not as likely a process but possible. However, O atom transfer can readily account for a two-electron transfer in MnO2 reduction. Better knowledge of the structures of Mn intermediates and of the types of reductant appears to be key for describing whether two one-electron transfer steps or a single two-electron step may be operative during MnO2 reduction.