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.