Electronic spectra of M(CO)6 (M = Cr, Mo, W) revisited by a relativistic TDDFT approach

Relativistic time dependent density functional calculations have been performed on the excited states of the M(CO)(6) (M = Cr, Mo, W) series. Our results, in agreement with previous density functional(1) and ab initio(2) calculations on Cr(CO)(6), indicate that in all members of the series the lowest excited states in the spectra do not correspond to ligand field (LF) excitations, as has been accepted in the past. Instead they correspond to charge transfer (CT) states. The LF excitations are calculated at much higher energy than suggested by the original assignment by Beach and Gray(3) and at different energy along the M(CO)a series, being much higher in the heavier carbonyls than in Cr(CO)(6). These results lead to a definitive reassessment of the role of the LF states in the photochemical dissociation of the metal-CO bonds in the M(CO)(6) series, suggesting that the experimentally observed photodissociation of the M-CO bond upon irradiation into the lowest energy bands occurs in the heavier carbonyls, as it does in Cr(CO)(6), from CT and not from LF states. A comparison with the experimental data available and, in the case of Cr(CO)(6), also with high-level correlated ab initio calculations(2) proves the reliability of the present TDDFT approach. The choice of the exchange-correlation (XC) functional is found to have a large effect on the excitation energies, demonstrating that even for quite "normal", low-lying excitations the XC functional may play an important role. In the heavier carbonyls, mostly in W(CO)(6), relativistic effects are seen to be relevant for the LF states as well as for the CT states arising from the (2t(2g))(5)(3t(2g))(1) configuration.