Supramolecular mixed valence assemblies exhibiting picosecond rates of electron transfer are reported. Trinuclear ruthenium clusters of the type [Ru3O(OAc)6(CO)(L)(pyS)], where L = 4-cyanopyridine, pyridine, and 4-dimethylaminopyridine, bound to Au nanoparticles (NPs) are electrochemically reduced to form mixed valence nanoclusters. Infrared spectroelectrochemical responses show dynamic coalescence of the ν(CO) region, indicating that electron transfer (ET) between Ru clusters attached via a π-conjugated bridge to the NPs occurs on the vibrational time scale. Bloch simulation of IR ν(CO) spectra gives ket on the order of 1011–1012 s–1. Ground state electron transfer rates exhibit a strong dependence on solvent dynamics. Results suggest that solvent dynamics control electron transfer, while solvent reorganization energy, λs, is apparently not influencing ET rates. This behavior is consistent with supramolecular mixed valence assemblies that lie on the nearly barrierless, Class II/III borderline of mixed valency.