Molecular structures and excited states of Cp(CO)(2) (CP = eta(5)-C5H5; M = Rh, Ir) and [Cl2Rh(CO)(2)](-) complexes have been investigated using the B3LYP and the symmetry-adapted cluster (SAC)/SAC-configuration interaction (SAC-Cl) theoretical methods. All the dicarbonyl complexes have singlet ground electronic states with large singlet-triplet separations. Thermal dissociations of CO from the parent dicarbonyls are energetically unfavorable. CO thermal dissociation is an activation process for [Cl2Rh(CO)(2)](-) while it is a repulsive potential for CpM(CO)(2). The natures of the main excited states of CPM(CO)(2) and [Cl2Rh(CO)(2)](-) are found to be quite different. For [Cl2Rh(CO)(2)](-), all the strong transitions are identified to be metal to ligand CO charge transfer (MLCT) excitations. A significant feature of the excited states of CpM(CO)p is that both MLCT excitation and a ligand Cp to metal and CO charge transfer excitation are strongly mixed in the higher energy states with the latter having the largest oscillator strength. A competitive charge transfer excited state has therefore been identified theoretically for CpRh(CO)(2) and Cplr(CO)(2). The wavelength dependence of the quantum efficiencies for the photoreactions of CPM(CO)(2) reported by Lees et al. can be explained by the existence of two different types of excited states. The origin of the low quantum efficiencies for the C-H/Si-H bond activations of CPM(CO)(2) can be attributed to the smaller proportion of the MLCT excitation in the higher energy states.