Direct Evidence of a La-Te Bond and Bonding Regime Differences for Ln-C and Ln-Te in [K(18-Crown 6)(Cp″2LnTe3)] (Ln = La, Ce, and Nd).

Using nonclassical divalent lanthanide precursors as multielectron reducing agents, complexes [K(18-crown-6)(Cp″2LnTe3)] with Ln = La, Ce, and Nd; Cp″ = 1,3-bis(trimethylsilyl)cyclopentadienyl were synthesized and investigated to elucidate the mechanism of lanthanide-tellurium bonding and the role of 4f-element electron density in stabilizing small chalcogenide chains. Density functional theory (DFT) reveals that the frontier molecular orbitals of these complexes are predominantly localized on the [Te3]2- fragment, while the trivalent lanthanide ions stabilize the tellurium chain through weak but measurable metal-ligand interactions. To experimentally resolve these interactions, we focus on complementary ligand- and metal-centered X-ray spectroscopic approaches. Ln L3-edge high-resolution XANES (HR-XANES) and valence-band resonant inelastic X-ray scattering (VB-RIXS) demonstrate that the Ln-Te/C interaction has substantial Ln 5d orbital contributions, particularly for the Ln-Te bond, and remain largely constant across the three complexes. DFT-based bond analysis provides a mechanistic interpretation of these trends. The Ln-C interaction exhibits increasing electron density at the bond critical point and a higher delocalization index (QTAIM analysis) from the lighter to the heavier lanthanides, reflecting enhanced Ln 4f participation within an energy-driven covalency regime. The Ln-Te interaction is predominantly electrostatic, with meaningful orbital contributions arising mainly from Ln 5d participation within an orbital-overlap-driven covalency regime. These results demonstrate that the Ln-C and Ln-Te bonding in the present complexes follow fundamentally distinct covalency mechanisms, which together enable the stabilization and isolation of the small [Te3]2- fragment chain.