A CASSCF/CASPT2 and TD-DFT Study of the Low-Lying Excited States of η₅-CpMn(CO)₃

Abstract

The electronic and geometric structures of η5-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 b1A‘ corresponding to a 3dMn → 3dMn excitation calculated at 25 733 cm-1 (3.22 eV) and the d1A‘ 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 c1A‘ (3dMn → 3dMn) 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 1A‘ ‘ (3dMn → 3dMn) 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 (3dMn → π*CO) are calculated between 37 410−45 019 cm-1 and are well separated from the metal centered (MC) (3dMn → 3dMn) states (≈1.0 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.