Multireference ab initio study of the ground and low-lying excited states of Cr(CO)2 and Cr(CO)3.

We investigate the ground and low-lying excited states of unsaturated chromium carbonyls, Cr(CO)2 and Cr(CO)3, using multiconfigurational ab initio perturbation theory. Unlike other chromium carbonyls, there are discrepancies between the experiment and theory on the identity of the ground states of Cr(CO)2 and Cr(CO)3. From multireference ab initio calculations considering the full valence orbitals of Cr(CO)2 and Cr(CO)3, the differences in the molecular structures of their various electronic states are explained by the electronic structure analysis. On the basis of the result from CASPT2 and MS-CASPT2 calculations, we propose that the ground states of Cr(CO)2 and Cr(CO)3 are the (5)Πg and (1)A1 states, respectively, addressing the ambiguity regarding their ground states. In addition, the multiconfigurational ab initio perturbation theory calculations reveal that (1) the energy gaps between the ground and first low-lying excited states of Cr(CO)2 and Cr(CO)3 are quite small and (2) the first low-lying excited states of Cr(CO)2 and Cr(CO)3 have the same spin multiplicities as the ground states of CrCO and Cr(CO)2, respectively, which are the products of ligand dissociation. As a result, the apparent spin-forbidden dissociation of Cr(CO)2 and Cr(CO)3 into CrCO and Cr(CO)2, respectively, are likely to be facilitated by thermal excitation of the ground states of Cr(CO)2 and Cr(CO)3 into their first low-lying excited states, which then actually undergoes the spin-allowed dissociation to the ground states of CrCO and Cr(CO)2 with the same spin multiplicities.