Cr-CO photodissociation in Cr(CO)(6): Reassessment of the role of ligand-field excited states in the photochemical dissociation of metal-ligand bonds

Density functional calculations have been performed on the excited states of Cr(CO)(6). In contrast to the generally accepted assignment of the spectrum by Gray and Beach(1,2) but in agreement with recent CASSCF/CASPT2 calculations by Pierloot et al.(3) we find the low-intensity absorption at the low-energy side of the first charge-transfer (CT) band not to be due to ligand-field (LF) excited states, but to symmetry forbidden CT excitations. In Cr(CO)(6) as in other d(6) metal-carbonyl complexes,(4-6) the LF states are at high energy. The calculations show that two states arising from the low-energy CT configuration have dissociative potential energy surfaces, in agreement with the experimentally observed photodissociation of the Cr-CO bond upon low-energy absorption. The photodissociation is therefore occurring from CT and not from LF states. This leads to a reassessment of the role of LF states in metal-ligand photodissociation: it is not necessary to excite to LF states in order to Induce photodissociation of ligands, and such dissociation, when observed, does not prove that the excitation was to a LF state.