Ligand-centered oxidation of manganese(II) complexes

Abstract

The oxidation potentials for an extensive group of manganese(II) complexes and their zinc(II) analogues have been determined by cyclic voltammetry in acetonitrile. With the exception of the pyridine, pyridine N-oxide, and H{sub 2}O ligands, the oxidations of the manganese(II) complexes occur at substantially less positive potentials than those for the zinc(II) analogous and clearly are ligand-centered. The removal of an electron from the valence shell of the ligand is easier than removal from the d{sup 5} manifold of manganese(II) and is facilitated by formation of a manganese (d-electron)-ligand (p-electron) covalent bond. The negative shift in potential for ligand oxidation is proportional to the bond energy. The near-edge x-ray absorption energies for the oxidized and reduced forms of the manganese complexes have been determined and correlated with the electrochemical results. The edge energy for manganese(II) is 6546.0 {plus minus} 0.3 eV, for manganese(II), 6550 {plus minus} 0.5 eV, and for the manganese(II) covalently bonded to an oxidized ligand, 6548.3 {plus minus} 0.5 eV. A manganese(II/III) valence change has a shift of a 4.0 eV, and the formation of a manganese(II)-({sup {sm bullet}}L) covalent bond from a ligand-centered oxidation has an average shift of 2.3 eV per bond formed. Formulation of MnO{submore » 4}{sup {minus}} as Mn{sup II}({sup {sm bullet}}O{sup {sm bullet}})({sup {sm bullet}}O{sup {minus}}){sub 3}{sup {minus}} with five Mn-O covalent bonds leads to a predicted edge energy of 6557.5 eV; the observed value is 6556.5 eV.« less