Four different redox states of the trinuclear ruthenium clusters of general formula [Ru-3(mu(3)-O)(mu-CH3CO2)(6)(L)(3)] (L = xylyl isocyanide (1), tert-butyl isocyanide (2)) were generated electrochemically in solution and studied by reflectance infrared spectroelectrochemistry. The IR spectra in the nu-(C drop N) region show multiple-band structure in the (RuRu2II)-Ru-III (-1) and (Ru2RuII)-Ru-III (0) states but a sharp single band in the Ru-3(III) (+1) Or (RuRu2III)-Ru-IV (+2) State. Density functional theory (DFT) ab initio calculations were performed for free xylyl isocyanide and tert-butyl isocyanide. The results suggest that the C drop N-C bending mode interacts through Fermi resonance with the nu(C drop N) fundamental for the 0 and -1 states of both 1 and 2. The extensive redox chemistry of the RU3- (mu(3)-O) clusters 1 and 2 offers a convenient means of effecting shifts in the energy of mu(C drop N) bands of coordinated isocyanide ligands, in and out of Fermi resonance with the C drop N-C bending overtone.