The duality of electron localization and covalency in lanthanide and actinide metallocenes

Previous magnetic, spectroscopic, and theoretical studies of cerocene, Ce(C₈H₈)₂, have provided evidence for non-negligible 4f-electron density on Ce and implied that charge transfer from the ligands occurs as a result of covalent bonding. Strong correlations of the localized 4f-electrons to the delocalized ligand π-system result in emergence of Kondo-like behavior and other quantum chemical phenomena that are rarely observed in molecular systems. In this study, Ce(C₈H₈)₂ is analyzed experimentally using carbon K-edge and cerium M_5,4-edge X-ray absorption spectroscopies (XAS), and computationally using configuration interaction (CI) calculations and density functional theory (DFT) as well as time-dependent DFT (TDDFT). Both spectroscopic approaches provide strong evidence for ligand → metal electron transfer as a result of Ce 4f and 5d mixing with the occupied C 2p orbitals of the C₈H₈ ^2- ligands. Specifically, the Ce M_5,4-edge XAS and CI calculations show that the contribution of the 4f¹, or Ce³⁺, configuration to the ground state of Ce(C₈H₈)₂ is similar to strongly correlated materials such as CeRh₃ and significantly larger than observed for other formally Ce⁴⁺ compounds including CeO₂ and CeCl₆ ^2-. Pre-edge features in the experimental and TDDFT-simulated C K-edge XAS provide unequivocal evidence for C 2p and Ce 4f covalent orbital mixing in the δ-antibonding orbitals of e_2u symmetry, which are the unoccupied counterparts to the occupied, ligand-based δ-bonding e_2u orbitals. The C K-edge peak intensities, which can be compared directly to the C 2p and Ce 4f orbital mixing coefficients determined by DFT, show that covalency in Ce(C₈H₈)₂ is comparable in magnitude to values reported previously for U(C₈H₈)₂. An intuitive model is presented to show how similar covalent contributions to the ground state can have different impacts on the overall stability of f-element metallocenes.