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Previous magnetic, spectroscopic, and theoretical studies of cerocene, Ce(C 8 H 8 ) 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(C 8 H 8 ) 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 C 8 H 8  2- ligands. Specifically, the Ce M 5,4 -edge XAS and CI calculations show that the contribution of the 4f 1 , or Ce 3+ , configuration to the ground state of Ce(C 8 H 8 ) 2 is similar to strongly correlated materials such as CeRh 3 and significantly larger than observed for other formally Ce 4+ compounds including CeO 2 and CeCl 6  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(C 8 H 8 ) 2 is comparable in magnitude to values reported previously for U(C 8 H 8 ) 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.

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