Light Lanthanide Metallocenium Cations Exhibiting Weak Equatorial Anion Interactions

As the dysprosocenium complex [Dy(Cpttt)2][B(C6F5)4] (Cpttt=C5H2tBu3‐1,2,4, 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(Cpttt)2][B(C6F5)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 LnIII cations, that contact ion pairs [Ln(Cpttt)2{(C6F5‐κ1‐F)B(C6F5)3}] (1‐Ln; La–Nd) are isolated from reactions of parent [Ln(Cpttt)2(Cl)] (2‐Ln) with [H(SiEt3)2][B(C6F5)4], where the anion binds weakly to the equatorial sites of [Ln(Cpttt)2]+ through a single fluorine atom in the solid state. For smaller SmIII, [Sm(Cpttt)2][B(C6F5)4] (1‐Sm) is isolated, which like heavier 1‐Ln does not exhibit equatorial anion interactions, but the EuIII analogue 1‐Eu could not be synthesised due to the facile reduction of EuIII precursors to EuII 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. Metal sandwiches: A family of light lanthanide metallocenium cations is reported. For the largest members of this series, equatorial interactions with a weakly coordinating anion are observed.