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 [C4(SiMe3)4]2- (Cb'''') produce the half-sandwich complexes [Na(12-crown-4)2][U(η4-Cb'''')(BH4)3] and [U(η4-Cb'''')(μ-BH4)3{K(THF)2}]2. In the 1 : 2 reaction of U(BH4)4 with Na2Cb'''', formation of [U(η4-Cb'''')(η3-C4H(SiMe3)3-κ-(CH2SiMe2)(BH4))]- 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.