Fast time-resolved infrared spectroscopy (TRIR) is used to probe the nu(CO) spectrum of ClRe(CO)3(4,4’-bipyridyl)2 in the lowest MLCT (metal-to-ligand charge transfer) excited state. The three ground-state nu(CO) bands all shift up in frequency, but not by the same amount. The energy-factored force field (EFFF) of the ground state is accurately solved by employing (CO)-C-13-enrichment. Approximate methods yield information on the force field of the excited state; the principal EFFF force constants (k(CO(axial)) and k(CO(equatorial))) change by the same amount on excitation. This leads to a new interpretation of the resonance Raman spectrum of the compound.