Quantitative photochemistry and mechanisms for a series of rhodium dicarbonyl derivatives

A comparison of the electronic properties and photochemical reactivity effi ciencies for a series of rhodium dicarbonyl complexes with the general form ula 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 b een achieved. The photochemical reactions of these molecules have been stud ied under a variety of experimental conditions, where binucleation, intermo lecular Si-H and C-H bond activation, and ligand substitution processes occ ur. Modifications of the unique ligand in this system enable the photoeffic iency of the CO dissociation reaction to be substantially changed. The abso lute reaction quantum efficiencies (phi(cr)) are markedly different and str ongly 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 de pending on the complex chosen and the excitation wavelength employed. The r esults obtained indicate that the photoefficiencies of these complexes are predominantly determined by radiative and nonradiative decay processes of n eighboring excited states and the electronic and steric properties of the u nique ligand.