A CASSCF/CASPT2 and TD-DFT study of the low-lying excited states of η5-CpMn(CO)3

The electronic and geometric structures of eta (5)-CpMn(CO)(3) in the near-UV region are investigated through CASSCF/CASPT2 and TD-DFT methods. The optimized geometries obtained at different levels of calculation are compared to the crystal and gas-phase structures for the electronic ground state. The change of geometry when going from the electronic ground state to the low-lying excited states was analyzed on the basis of gradient-CASSCF calculations. The lowest excited-state b(l)A' corresponding to a 3d(Mn) --> 3d(Mn) excitation calculated at 25 733 cm(-1) (3.22 eV) and the d(l)A' calculated at 30 366 cm(-1) (3.80 eV) with very low oscillator strengths (<0.007) do not show any significant geometry changes with respect to the electronic ground state. The main geometry changes which never exceed 10% correspond to elongations of the Mn-Cp and Mn-CO bonds (with the out-of-plane CO ligands), The c(l)A' (3d(Mn) --> 3d(Mn)) absorbing state calculated at 26 470 cm(-1) (3.31 eV) with an oscillator strength of 0.0157 is characterized by an elongation of the Mn-COax bond (COax being the in-plane carbonyl) and does not converge to a minimum, which is a characteristic of dissociative states. Among the (l)A (3d(Mn) --> 3d(Mn)) states calculated between 24 972 and 29 949 cm(-1) only the lowest one has an oscillator strength exceeding 0.01. The metal to ligand charge transfer (MLCT) states (3d(Mn) --> pi*(CO)) are calculated between 37 410-45 019 cm(-1) and are well separated from the metal centered (MC) (3d(Mn) --> 3d(Mn)) states (approximate to1.0 eV). The time-dependent DFT excitation energies and related assignments compare rather well to the multistate-CASPT2 results as far as the lowest MC excited states are concerned.