Atomic Orbital Interactions in [CeX6]2– (X = F, Cl, Br): Consequences for X-Ray Absorption Spectroscopic Analysis

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Metal–ligand bonding in [CeX 6 ] 2– complexes (X = F, Cl, Br) is investigated using density functional theory (DFT) and pre-bonding analysis. The analysis quantifies atomic orbital (AO) energies and overlaps in the complexes, with a focus on the mixing between Ce 4f AOs with halide 𝑛p donor AOs. Results indicate that decreasing Ce–ligand AO overlap from X = F to Br is compensated by improved AO energy matching, leading to increased 4f-based ligand-to-metal donation across the series. Population analysis derived from pre-bonding data rationalizes recent experimental X-ray absorption near-edge structure (XANES) findings. Effective orbital overlaps extracted from XANES data using a simplified AO bonding model based on the Wolfsberg–Helmholtz (WH) relation are compared directly with pre-bonding overlaps. The consequences of using ionization potential (IP) based AO energies in the experimental data workup and other assumptions inherent in the bonding model are also scrutinized. Using consistent pre-bonding AO energies in the model requires a WH constant of 𝐾 ≃ 3 to reproduce the calculated overlaps for 4f-based interactions. Combining IP-based AO energies with a conventional value 𝐾 = 2, as done in prior studies, is shown to yield comparable overlaps due to a cancellation of errors. Overall, pre-bonding analysis provides a framework rooted in first-principles theory for connecting AO interactions in the molecule with experimental bonding trends.

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