Synthesis and X-ray Crystal Structure of [U(η-C8H8)(HMPA)3][BPh4], the First Cationic Monocyclooctatetraenyl Compound of Uranium (III), and its Neodymium Homologue

The title compound was synthesized in 88% yield by reducing the uranium(IV) dication [U(COT)(HMPA)3][BPh4]2 (COT = η-C8H8) with sodium amalgam while the neodymium analogue was prepared by substitution of the THF ligands in [Nd(COT)(THF)4][BPh4]. Comparison of the crystal structures of the complexes [M(COT)(HMPA)3][BPh4] (M = U, Nd, Sm) shows that the variations in the M−ligand bond lengths simply reflect the differences in the ionic radii of the metals.