A multi state-CASPT2 vs. TD-DFT study of the electronic excited states of RCo(CO)4 (R = H, CH3) organometallic complexes

The electronic spectroscopy of RCo(CO)4 (R = H, CH3) has been investigated by means of MS-CASPT2/CASSCF and TD-DFT calculations. The UV energy domain of the absorption spectrum is dominated by a series of low-lying 1E metal-to-sigma-bond-charge-transfer (MSBCT) and 1E and 1A1 metal-to-ligand-charge-transfer (MLCT) allowed transitions corresponding to 3 d Co → σ Co – R ∗ and 3 d Co → π CO ∗ excitations, respectively, slightly red shifted (by ca. 1800 cm−1) in the methyl substituted complex. TD-DFT underestimates the transition energies calculated by MS-CASPT2 by more than 0.5 eV and predicts some of the MLCT states to be intercalated between the two low-lying MSBCT states.