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)

Abstract 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.