Ligand-Field Excited States of Metal Hexacarbonyls

Over 35 years ago, the low-lying bands in the absorption spectra of metal hexacarbonyls were assigned to ligand-field (LF) excitations. Recent time-dependent density functional theory (TDDFT) calculations on M(CO)_6 (M = Cr, Mo, W) are not in accord with this interpretation. Here we extend TDDFT calculations to the isoelectronic series V(CO)_6^-, Cr(CO)_6, and Mn(CO)_6^+. By analyzing the trends in the energies of the various electronic excitations, we are able to fully assign the spectra of the complexes. In particular, we demonstrate that the LF excitation ^1A_(1g) → ^1T_(1g) is observed at 4.12 eV in the Mn(CO)_6^+ spectrum, but all LF features in the spectra of V(CO)_6^- and Cr(CO)_6 are obscured by intense metal-to-ligand charge-transfer absorptions. Our results suggest that use of B3LYP as the exchange-correlation functional and inclusion of solvation effects through a continuum solvation model lead to the most accurate calculated transition energies.