Manganese mixed oxidation state complexes

Manganese oxides in natural systems are often complex minerals characterized by poor crystallinity and mixed oxidation states (McKenzie, 1989). In addition to being electron acceptors in the oxidation of As(III) to As(V), Mn oxides can also adsorb As. Direct evidence for adsorption of As by Mn oxides comes from laboratory experiments in which pure mineral phases have been used. 

It is interesting to note that these complexes are mixed-valent MnmMnIV complexes. Based on the relative structural data [the bond distances of the MnA atom are shorter than those of MnB], it has been concluded that in [Mn202(bipy)4]3+ one of the manganese ions is in the oxidation state IV [Mn(B)] and the other in the oxidation state III [Mn(A)]. Hence, the complex would have to be classified as a mixed-valent derivative with localized charge (Robin-Day Class I). Conversely, the two manganese sites are identical in [Mn202(phen)4]" +, from which one can infer that the charge is delocalized over the two centres (Robin-Day Class III). 

Manganese forms a large number of mononuclear complexes in oxidation states II, III, and IV with acyclic and other common ligands. The complexes primarily are six-coordinate species with octahedral or pseudo-octahedral geometries. Some exceptions to this categorization are the tetraoxo anions in oxidation states V, VI, and VII and the manganocenes. Table 1 contains electrochemical data for representative redox couples in which the Mn coordination environment remains invariant with oxidation state. Many entries are for homoleptic complexes, but some species with mixed... 

STUDIES OF MIXED VALENCE MANGANESE COMPOUNDS Mn304 is a commercial product which has a normal spinal structure with one Mn(ll) and two Mn(III) ions per formula. However, some complexes contain one Mn(III) and one Mn(IV) per formula and they are also considered mixed valence complexes. Most of the reactions of these compounds are oxidations in which Mn is reduced to the most stable state of +2. 

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