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 LiC6Cl5 in 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 C6Cl5 ligands. 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-Cipso donation bonds feature only weak 4f participation (e.g., ∼1% 4f weight for [1]-). Nonetheless, the 13C chemical shift of the Cipso nuclei 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.