Mixed Valence Dinuclear Species

The Creutz–Taube ion, (NH₃)₅Ru(μ-pyrazine)Ru(NH₃)₅⁵⁺, was one of the earliest intentionally prepared molecular mixed-valence complexes.^1,2 Initially formulated as Ru^II-bridge-Ru^III, it was intended by its designers to provide an example of the direct experimental assessment of the reorganization energy for an intramolecular electron transfer reaction—in this case: (Formula presented) Briefly, the idea was to use Hush’s theory relating optical intervalence transfer (Equation (2)) to thermal electron transfer (Equation (1)).³ As summarized in Figure 1 for a symmetrical system (Formula presented) an electronic absorption is expected at an optical energy equaling the total reorganization energy, and also equaling four times the activation free energy for nonadiabatic electron transfer. Hush had previously shown that the theory could be applied to mixed-valent mineral compounds, with optical intervalence transitions accounting in many cases for the colors of the minerals.⁴ Before Creutz and Taube’s report, however, the theory hadn’t been applied to well-defined symmetrical molecular systems in solution environments. As shown in Figure 2, the Creutz–Taube (CT) ion yields the intervalence absorption band promised by Hush theory.^1,2 Furthermore, the band appears at an energy that seems reasonable as an electron-transfer reorganization energy. In other respects, however, the observations are inconsistent with the Hush model. Most notably, the band is non-gaussian and is much narrower than expected from the model (which relates the width to the reorganization energy). Furthermore, it is relatively insensitive to solvent composition, with a residual sensitivity that differs from what is expected from solvent contributions to the reorganization energy. The observations....