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

Complexes of the form [U(η 6 − arene)(BH 4 ) 3 ] where arene = C 6 H 6 ; C 6 H 5 Me; C 6 H 3 -1,3,5-R 3 (R = Et, iPr, tBu, Ph); C 6 Me 6 ; and triphenylene (C 6 H 4 ) 3 were investigated towards an understanding of the nature of the uranium-arene interaction. Density functional theory (DFT) shows the in- teraction 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 or- bital 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 electro- static 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 C 6 Me 6 . Finally, both DFT and multireference calculations on a reduced congener, [U(C 6 Me 6 )(BH 4 ) 3 ] − – formally U(II), suggest 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. Instead, we show the structure strongly depends upon the spin-state of the complex but that in both spin-states it is best described as U(III) and an arene-centred radical.