Direct evidence of a lanthanide-tellurium bond in [K(18-crown 6)(Cp′′2LnTe3)] with Ln = La, Ce, and Nd

Using non-classical divalent lanthanide precursors as multi-electron 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 orbitals contribution, particularly for the Ln-Te bond, and remains 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. ... mehrThe 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.