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 4<i>f</i>-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.