Resonant Inelastic X-ray Scattering Covalency Analysis Reveals Non-Aufbau Electronic Configurations in Low-Valent Lanthanides

Divalent lanthanide ions can adopt complex 4fn5d1 electron configurations that formally violate the Aufbau principle. The blending of contracted 4f and diffuse 5d character strongly influences lanthanide frontier molecular orbitals, yet quantitative characterization of 5d electron participation in bonding remains challenging. To address this, we report L3-edge high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) and valence to core resonant inelastic X-ray scattering (V2C-RIXS) for a series of divalent lanthanide complexes [M(NHAriPr6)2] (M = Ce, Tb, Dy, and Yb; NHAr2iPr6 = N(H)C6H3-2,6-(C6H2-2,4,6-iPr3)2) and their trivalent precursors [M(NHAriPr6)2(I)], which all feature close metal–arene contacts. It is demonstrated that V2C-RIXS can be applied to quantify relative 5d covalency contributions to lanthanide–arene bonding. Analysis of divalent cerium, terbium, and dysprosium complexes reveals non-Aufbau (5d1) configurations, which show significant 5d–arene δ-bond covalency and stabilization of the associated bonding orbitals. V2C-RIXS spectra are accurately simulated and compared with UV–vis–NIR results using density functional theory (DFT)/restricted open configuration interaction with singles (ROCIS) calculations. A detailed assignment of spectral features and correlation to predicted electron configurations defines a trend of increased 5d covalency for these complexes relative to their trivalent precursors. Conversely, the divalent ytterbium complex has an Aufbau configuration, 4f14, and shows markedly reduced 5d covalency relative to its trivalent precursor.