Investigation of the electronic structures of organolanthanide sandwich complex anions by photoelectron spectroscopy: 4f orbital contribution in the metal-ligand interaction

The electronic structures of lanthanide (Ln) ions sandwiched between 1,3,5,7-cyclooctatetraene (COT), Ln(COT)₂^-, have been investigated by anion photoelectron spectroscopy. Complexes of 12 Ln atoms were investigated (excluding promethium (Pm), europium (Eu), and ytterbium (Yb)). The 213 nm photoelectron (PE) spectra of Ln(COT)₂^- exhibit two peaks assignable to the highest occupied molecular orbital (HOMO; e _2u) and the next HOMO (HOMO-1; e_2g) approximately at 2.6 and 3.6 eV, respectively, and their energy gap increases as the central metal atom progresses from lanthanum (La) to lutetium (Lu). Since lanthanide contraction shortens the distance between the Ln atom and the COT ligands, the widening energy gap represents the destabilization of the e_2u orbital as well as the stabilization of the e_2g orbital. Evidence for 4f orbital contribution in the metal-ligand interaction has been revealed by the Ln atom dependence in which the same e_2u orbital symmetry enables an interaction between the 4f orbital of Ln atoms and the π orbital of COT.