How the Oxidation State Drives Bonding Properties in CeIII and CeIV Tris(amidinate) Complexes

We report the synthesis and characterization of a pair of homoleptic cerium tris(amidinate) complexes, [CeIII{PhC(NtBu)2}3] and [CeIV{PhC(NtBu)2}3][Al(OC4F9)4], containing cerium in both its trivalent (CeIII) and tetravalent (CeIV) oxidation states. To the best of our knowledge, these represent the first examples of a lanthanide-amidinate system where both trivalent and tetravalent complexes have been isolated and structurally characterized in such a congruent ligand coordination. With electronic structure and bond analysis, we demonstrate that while the metal-ligand bonding in both species is dominated by interactions involving Ce 5d valence orbitals, there is a measurable participation of the 4f orbitals specifically within the CeIV homolog. Ce L3-edge high-energy resolution X-ray absorption near edge structure (HR-XANES) and valence band resonant inelastic X-ray scattering (VB-RIXS) are employed to evaluate the bonding properties and are complemented with quantum chemical calculations based on density-functional theory (DFT). Crucially, the high-resolution nature of these spectroscopic techniques, coupled with theoretical modelling, enables a quantitative deconvolution of the bonding interaction directly from the experimental data. ... mehrThis approach allows us to partition the individual contributions of both the 4f and 5d electrons to the metal-ligand interaction. Our findings highlight the increased role of 4f electron density in stabilizing high-valent lanthanide complexes, due to improved energetic and spatial overlap between nitrogen donor lone pairs and the CeIV acceptor manifold.