Light Lanthanide Metallocenium Cations Exhibiting Weak Equatorial Anion Interactions

As the dysprosocenium complex [Dy(Cp(ttt))(2)][B(C(6)F(5))(4)] (Cp(ttt)=C(5)H(2) tBu(3)‐1,2,4, 1‐Dy) exhibits magnetic hysteresis at 60 K, similar lanthanide (Ln) complexes have been targeted to provide insights into this remarkable property. We recently reported homologous [Ln(Cp(ttt))(2)][B(C(6)F(5))(4)] (1‐Ln) for all the heavier Ln from Gd–Lu; herein, we extend this motif to the early Ln. We find, for the largest Ln(III) cations, that contact ion pairs [Ln(Cp(ttt))(2){(C(6)F(5)‐κ(1)‐F)B(C(6)F(5))(3)}] (1‐Ln; La–Nd) are isolated from reactions of parent [Ln(Cp(ttt))(2)(Cl)] (2‐Ln) with [H(SiEt(3))(2)][B(C(6)F(5))(4)], where the anion binds weakly to the equatorial sites of [Ln(Cp(ttt))(2)](+) through a single fluorine atom in the solid state. For smaller Sm(III), [Sm(Cp(ttt))(2)][B(C(6)F(5))(4)] (1‐Sm) is isolated, which like heavier 1‐Ln does not exhibit equatorial anion interactions, but the Eu(III) analogue 1‐Eu could not be synthesised due to the facile reduction of Eu(III) precursors to Eu(II) products. Thus with the exception of Eu and radioactive Pm this work constitutes a structurally similar family of Ln metallocenium complexes, over 50 years after the [M(Cp)(2)](+) series was isolated for the 3d metals.