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

Relativistic time dependent density functional calculations have been perfo rmed on the excited states of the M(CO)(6) (M = Cr, Mo, W) series. Our resu lts, in agreement with previous density functional(1) and ab initio(2) calc ulations on Cr(CO)(6), indicate that in all members of the series the lowes t excited states in the spectra do not correspond to ligand field (LF) exci tations, as has been accepted in the past. Instead they correspond to charg e transfer (CT) states. The LF excitations are calculated at much higher en ergy 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 reassessmen t of the role of the LF states in the photochemical dissociation of the met al-CO bonds in the M(CO)(6) series, suggesting that the experimentally obse rved photodissociation of the M-CO bond upon irradiation into the lowest en ergy bands occurs in the heavier carbonyls, as it does in Cr(CO)(6), from C T 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 c alculations(2) proves the reliability of the present TDDFT approach. The ch oice of the exchange-correlation (XC) functional is found to have a large e ffect on the excitation energies, demonstrating that even for quite "normal ", low-lying excitations the XC functional may play an important role. In t he 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.