Mixed-valence compounds localization , oxidation states

X-Ray photoelectric ionization is believed to take place in a time interval of about 10-18 s. Therefore separate XPS peaks are possible for atoms if the lifetime of the asymmetric electronic state is greater than about 10 18 s, whether or not the atoms are structurally equivalent. We may represent the ground state of a localized mixed valence compound (involving two metal atoms differing in oxidation state by one unit) by the following formula, where the dot represents the extra valence electron M—M. The two possible XPS transitions can then be represented as follows, where the asterisk indicates core ionization,... 

It is well established that electron localization yields to a polarization of the intramolecular and extramolecular media. Thus, in coordination complexes, a change in oxidation state produces a change in metal ligand bond lengths. Consider for instance a binuclear mixed valence compound where an electron can exchange between two chemically equivalent sites. Since electronic transfer can occur only when the two metal sites have identical environments, a rearrangement of metal ligand bond distances must occur prior to electron transfer. It is then possible to compute in a semi-classical way the contribution... 

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