A study on the excitations of ligand-to-metal charge transfer in complexes Cp₂MCl₂ (Cp=π-C₅H₅, M=Ti, Zr, Hf) by density functional theory

The excitations of ligand-to-metal charge transfer in the UV-vis spectra of the complexes Cp₂MCl₂ (Cp=π-C₅H₅, M=Ti, Zr, Hf) were described with the time-dependent density functional theory and the differential self-consistent-field density functional theory (ΔSCF-DFT). The nature of the main spectral features was interpreted on the basis of the electronic structure of the complexes. The optimization of the molecular structure for Cp₂MCl₂ yielded consistent pseudotetrahedral geometries. With the transition from ground state to excited state, the electrons transferred from Cp ligands to the central metals M, accompanied by an increase in the binding length of M-Cl. This transfer process accounted for the first two absorption bands observed experimentally. The energies and oscillator strengths for the low-lying excitations depended strongly on the nature of the central metals. With varying the central metals from Ti, Zr, and to Hf, the energy gap increased between the sets of highest occupied molecular orbitals (HOMOs) and the sets of lowest unoccupied molecular orbitals (LUMOs). This increase was originated from an increment of energy for the LUMO sets that are mainly of d orbital of the metal and a relatively stable energy level of the HOMO sets that are mainly of p-orbital of Cp and Cl ligands. The excitation energies calculated by these methods showed a small difference, which can be well understood by a relativistic effect. The predicted level patterns of the lowest triplet excited states fit well the available phosphorescence data.