Divalent Lanthanide Metallocene Complexes with a Linear Coordination Geometry and Pronounced 6s-5d Orbital Mixing

A small but growing number of molecular compounds have been isolated featuring divalent lanthanides with 4f5d¹ electron configurations. While the majority of these possess trigonal coordination geometries, we previously reported the first examples of linear divalent metallocenes Ln(Cp^iPr5)₂ (Ln = Tb, Dy; Cp^iPr5 = pentaisopropylcyclopentadienyl). Here, we report the synthesis and characterization of the remainder of the Ln(Cp^iPr5)₂ (1-Ln) series (including Y and excluding Pm). The compounds can be synthesized through salt metathesis of LnI₃ and NaCp^iPr5 followed by potassium graphite reduction for Ln = Y, La, Ce, Pr, Nd, Gd, Ho, and Er, by in situ reduction during salt metathesis of LnI₃ and NaCp^iPr5 for Ln = Tm and Lu, or through salt metathesis from LnI₂ and NaCp^iPr5 for Ln = Sm, Eu, and Yb. Single crystal X-ray diffraction analyses of 1-Ln confirm a linear coordination geometry with pseudo-D₅ symmetry for the entire series. Structural and ultraviolet-visible spectroscopy data support a 4f electron configuration for Ln²⁺ = Sm, Eu, Tm, and Yb and a 4f5d¹ configuration for the other lanthanides ([Kr]4d¹ for Y²⁺). Characterization of 1-Ln (Ln = Y, La) using electron paramagnetic resonance spectroscopy reveals significant s-d orbital mixing in the highest occupied molecular orbital and hyperfine coupling constants that are the largest reported to date for divalent compounds of yttrium and lanthanum. Evaluation of the room temperature magnetic susceptibilities of 1-Ln and comparison with values previously reported for trigonal Ln²⁺ compounds suggests that the more pronounced 6s-5d mixing may be associated with weaker 4f-5d spin coupling.