Cerium oxidation state and covalent 4f-orbital contributions in the ground state of bis(η8-pentalene)cerium

Abstract

The sandwich complex bis(η8-pentalene)cerium Ce(C8H6)2 was investigated by relativistic quantum chemical ab initio methods and the nature of the interaction between the Ce 4f and ligand π orbitals was analyzed. It is shown for a multi-configurational ground state wavefunction of the complete active space variety that by means of an orbital rotation in the space of the strongly interacting orbitals it is possible to generate nearly pure Ce 4f and ligand π orbitals without changing the total energy. These orbitals are occupied by one electron each, leading to an open-shell 4f1π1 ground state configuration. The admixture of the energetically higher closed-shell 4f0π2 configuration leads to a singlet ground state, making the complex another example for a molecular analog of a Ce(III)-based lattice Kondo system. A further analysis using local spin and occupation number fluctuations in a two-electron two-orbital description reveals that the Ce 4f-ligand π interaction corresponds to a significantly stretched covalent bond: orbital and configuration mixing coefficients as well as local spin and occupation number fluctuations are very similar to those of hydrogen dimer H2 at an interatomic distance of about two times the equilibrium distance.