Multiple Structures and Dynamics of [CpRu(CO)2]2 and [CpFe(CO)2]2 in Solution Revealed with Two-Dimensional Infrared Spectroscopy

Two-dimensional infrared (2DIR) spectroscopy is applied to both (Cp)(2)Fe-2(CO)(4) and its ruthenium analog (Cp)(2)Ru-2(CO)(4) in order to study the vibrational dynamics of these two systems. Combining the results of 2DIR spectroscopy and DFT calculations, the different structural forms of both the iron and the ruthenium complexes were characterized, furthering the previous assignment of the linear IR spectrum by determining the transition frequencies associated with the different isomeric forms. Monitoring the time-dependent amplitudes of the cross peaks enabled the observation of equilibrium energy transfer dynamics between different vibrational modes of the cis-B (Cp)(2)Fe-2(CO)(4) and the gauche-NB (Cp)(2)Ru-2(CO)(4) complexes. Treating the energy transfer as an equilibrium process, we extracted the rate constants associated with both the uphill and the downhill transfer of vibrational energy, finding that the difference in the rate constants of the two metal complexes maps to the difference in the energy gap between the two modes involved.