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 5–8 ), cyclobutadienyl (C 4 ) complexes remain exceedingly rare. Here, we report that reaction of [Li 2 {C 4 (SiMe 3 ) 4 }(THF) 2 ] ( 1 ) with [U(BH 4 ) 3 (THF) 2 ] ( 2 ) gives the pianostool complex [U{C 4 (SiMe 3 ) 4 }(BH 4 ) 3 ][Li(THF) 4 ] ( 3 ), where use of a borohydride and preformed C 4 ‐unit circumvents difficulties in product isolation and closing a C 4 ‐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 4 ‐unit, and the SiMe 3 groups are displaced from the C 4 ‐plane, which we propose maximises U−C 4 orbital overlap. DFT calculations identify two quasi‐degenerate U−C 4 π‐bonds utilising the ψ 2 and ψ 3 molecular orbitals of the C 4 ‐unit, but the potential δ‐bond using the ψ 4 orbital is vacant.