What is the nature of the uranium(III)–arene bond?

Complexes of the form [U(η⁶-arene)(BH₄)₃] where arene = C₆H₆; C₆H₅Me; C₆H₃-1,3,5-R₃ (R = Et, iPr, tBu, Ph); C₆Me₆; and triphenylene (C₆H₄)₃ were investigated towards an understanding of the nature of the uranium–arene interaction. Density functional theory (DFT) shows the interaction energy reflects the interplay between higher energy electron rich π-systems which drive electrostatic contributions, and lower energy electron poor π-systems which give rise to larger orbital contributions. The interaction is weak in all cases, which is consistent with the picture that emerges from a topological analysis of the electron density where metrics indicative of covalency show limited dependence on the nature of the ligand – the interaction is predominantly electrostatic in nature. Complete active space natural orbital analyses reveal low occupancy U–arene π-bonding interactions dominate in all cases, while δ-bonding interactions are only found with high-symmetry and electron-rich C₆Me₆. Finally, both DFT and multireference calculations on a reduced, formally U(II), congener, [U(C₆Me₆)(BH₄)₃]⁻, suggests the electronic structure (S = 1 or 2), and hence metal oxidation state, of such a species cannot be deduced from structural features such as arene distortion alone. We show that arene geometry strongly depends on the spin-state of the complex, but that in both spin-states the complex is best described as U(III) with an arene-centred radical.