f-Element Half-Sandwich Complexes: A Tetrasilylcyclobutadienyl-Uranium(IV)-Tris(tetrahydroborate) Anion Pianostool Complex.

Despite there being numerous examples of f-element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C ), cyclobutadienyl (C ) complexes remain exceedingly rare. Here, we report that reaction of [Li {C (SiMe ) }(THF) ] (1) with [U(BH ) (THF) ] (2) gives the pianostool complex [U{C (SiMe ) }(BH ) ][Li(THF) ] (3), where use of a borohydride and preformed C -unit circumvents difficulties in product isolation and closing a C -ring at uranium. Complex 3 is an unprecedented example of an f-element half-sandwich cyclobutadienyl complex, and it is only the second example of an actinide-cyclobutadienyl complex, the other being an inverse-sandwich. The U-C distances are short (av. 2.513 Å), reflecting the formal 2- charge of the C -unit, and the SiMe groups are displaced from the C -plane, which we propose maximises U-C orbital overlap. DFT calculations identify two quasi-degenerate U-C π-bonds utilising the ψ and ψ molecular orbitals of the C -unit, but the potential δ-bond using the ψ orbital is vacant.