Quantitative photochemistry and mechanisms for a series of rhodium dicarbonyl derivatives

A comparison of the electronic properties and photochemical reactivity efficiencies for a series of rhodium dicarbonyl complexes with the general formula XRh(CO)(2), where X = Cp (eta(5)-C5H5), Cp' (eta(5)-C5H4Me), Cp " (eta(5)-C5HMe4), Cp* (eta(5)-C5Me5), ind (eta(5)-C9H7), and acac (C5H7O2), has been achieved. The photochemical reactions of these molecules have been studied under a variety of experimental conditions, where binucleation, intermolecular Si-H and C-H bond activation, and ligand substitution processes occur. Modifications of the unique ligand in this system enable the photoefficiency of the CO dissociation reaction to be substantially changed. The absolute reaction quantum efficiencies (phi(cr)) are markedly different and strongly wavelength dependent for each of the molecules studied. The values of phi(cr) decrease in the order CpRh(CO)(2) > Cp'Rh(CO)(2) > Cp*Rh(CO)(2) >> (acac)Rh(CO)(2) > (ind)Rh(CO)(2) and vary by over 3 orders of magnitude depending on the complex chosen and the excitation wavelength employed. The results obtained indicate that the photoefficiencies of these complexes are predominantly determined by radiative and nonradiative decay processes of neighboring excited states and the electronic and steric properties of the unique ligand.