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

Abstract

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.