4f-Orbital Occupancy in Tetravalent Cerium Halide Complexes from X-ray Spectroscopy and Theory

Recent studies of lanthanide bonding have shown that small amounts of metal-ligand covalency can impact bond energies and cause magnetic behavior to deviate from the free ion values. Quantifying covalent bonding parameters is necessary to support the rational tuning of these properties in applications for lanthanides in energy and quantum information science. Here, two measurements of f-electron occupancy in CeBr₆^2- are reported based on X-ray spectra at the Ce L₃- and Br K-edges and compared to the corresponding results for CeF₆^2- and CeCl₆^2-. These values allow for the estimation of average metal-ligand overlap integrals and covalent bond stabilization energies. For this series of compounds, overlap integrals and bond covalency are found to have opposing trends as the halide is varied. Finally, correlations between Ce^IV L₃-edge doublet splittings, 4f-orbital occupancies (n_f), and ligand-to-metal charge transfer (LMCT) energies are identified that may indicate significant mixing between the 4f and 5d orbitals in other, noncentrosymmetric Ce^IV complexes.