Manganese(II) Oxidation and Mn(IV) Reduction in the Environment—Two One-Electron Transfer Steps Versus a Single Two-Electron Step
The chemistry of electron transfer processes are reviewed using a knowledge of orbital properties and available experimental data. Fe(H 2 O) 6 2+ and Mn(H 2 O) 6 2+ oxidation with O 2 are discussed and compared. Mn(H 2 O) 6 2+ oxidation by O 2 occurs via an inner sphere process after complexation with inorganic (e.g.; OH - , increase pH) or organic ligands that replace water; whereas Fe(H 2 O) 6 2+ at circumneutral pH occurs via an outer sphere mechanism. An outer sphere electron transfer process is symmetry forbidden for Mn(H 2 O) 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(H 2 O) 6 2+ oxidation as hydroxide and organic ligands replace water and bind Fe(II). The bonding of O 2 to Fe(H 2 O) 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 O 2 to the metal with a bent M-O-O bond angle. Side-on bonding for O 2 to the metal is possible and could lead to two-electron transfers from Mn(II) compounds but requires stabilization of the Mn-O 2 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 O 2 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(H 2 O) 6 2+ by O 2 and the reduction of MnO 2 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(H 2 O) 6 2+ or as an electron donor forming insoluble MnO 2 . 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 x2-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 MnO 2 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 MnO 2 reduction.
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Document Type: Research Article
Affiliations: College of Marine Studies, University of Delaware, Lewes, Delaware, 19958, USA
Publication date: 2005-04-01