Uranium(iv) cyclobutadienyl sandwich compounds: synthesis, structure and chemical bonding.

The 1 : 1 reactions of uranium(iv) tetrakis(borohydride) with the sodium and potassium salts of the cyclobutadienyl anion [C(SiMe)] (Cb'''') produce the half-sandwich complexes [Na(12-crown-4)][U(η-Cb'''')(BH)] and [U(η-Cb'''')(μ-BH){K(THF)}]. In the 1 : 2 reaction of U(BH) with NaCb'''', formation of [U(η-Cb'''')(η-CH(SiMe)-κ-(CHSiMe)(BH))] reveals that a Cb'''' ligand undergoes an intramolecular deprotonation, resulting in an allyl/tuck-in bonding mode. A computational study reveals that the uranium-Cb'''' bonding has an appreciable covalent component with contributions from the uranium 5f and 6d orbitals.