Assessing the 4f Orbital Participation in the Ln-C Bonds of [Li(THF)₄][Ln(C₆Cl₅)₄] (Ln = La, Ce)

The reaction of [Ln(NO₃)₃(THF)₄] (Ln = La, Ce) with 4 equiv of LiC₆Cl₅in Et₂O resulted in the formation of the homoleptic lanthanide-aryl "ate" complexes [Li(THF)₄][La(C₆Cl₅)₄] ([Li][1]) and [Li(THF)₄][Ce(C₆Cl₅)₄] ([Li][2]). These complexes represent the first isolated homoleptic perchlorophenyl complexes for the lanthanides. In the solid state, both [Li][1] and [Li][2] exhibit octa-coordinate lanthanide centers, with four Ln-C σ-bonds and four Cl → Ln dative interactions involving the ortho-Cl atoms of the C₆Cl₅ligands. Despite this apparent steric saturation, both [Li][1] and [Li][2] are highly temperature sensitive and quickly decompose in solution at room temperature. Density functional calculations show that the Ln-C_ipsodonation bonds feature only weak 4f participation (e.g., ∼1% 4f weight for [1]^-). Nonetheless, the ¹³C chemical shift of the C_ipsonuclei of [1]^-includes ca. 8 ppm of deshielding from the spin-orbit interaction due to the participation of the 4f (and 5d) orbitals in the La-C bonds.