Single-electron reductions of linked triruthenium clusters of the general type Ru-3-pyrazine-Ru-3 produced mixed valence systems showing spectroscopic characteristics of rapid intramolecular electron transfer. Reflectance infrared spectroelectrochemistry was used to characterize the vibrational spectra of mixed valence systems that contained one carbon monoxide ligand on each Ru-3 cluster. Infrared spectra in the CO stretching region showed two resolved, partially coalesced, and coalesced nu(CO) bands for clusters with rate constants for intramolecular electron transfer k(e) increasing from = 1 x 10(9) s(-1) up to 5 x 10(11) and 9 x 10(11) s(-1), respectively. These data provide a strong correlation between rates of intramolecular electron transfer and infrared spectral bandshape.