Localized and Delocalized Mixed Valence

It is possible to make molecules or crystals of mixed valence system in different chemical ways. If two solutions with different valences are mixed, the chances are high that a mixed crystal is formed rather than two different crystals. It has also been possible to connect metal ions in different valence states with organic molecules. In the latter case, we may talk about a mixed valence molecule. 

If A X, there is only one minimum (Equation 10.27), and we have to consider the unpaired electron as physically delocalized. The valences are then the same and fractional. In the minimum at Q = 0, the electron is equally distributed over the two sites. Even for Q 9 0, the distribution is close to 50/50%. 

on the other hand, A X, there is no barrier and the excitation energy is equal to A. The absorption should be narrower and the energy less. The American chemists Carol Creutz and Henry Taube managed to synthesize a number of complexes where the metal ions Ru and Ru are connected by a bridge. 

FIGURE 10.7 Creutz-Taube mixed valence system (NH3)5Ru-pyrazine-Ru(NH3).  

Carol Creutz confirmed (1983) the delocalization by changing the polarity of the solvent. In the localized complexes, the spectrum depends almost linearly on X. However, this is not the case for the Creutz-Taube complex. Thus, the charges in this bimetallic complex are delocalized, as has been found in a number of experiments. A number of additional experiments to find out if the Creutz-Taube ion is really delocalized have been carried out. Reymers and Hush have discussed these experiments with the conclusion that the Creutz-Taube ion is, in fact, delocalized. This means that the oxidation states of Ru atoms are the same and equal to -t-2.5. 

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