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

Complexes of the form [U(eta(6)-arene)(BH4)(3)] where arene = C6H6; C6H5Me; C6H3-1,3,5-R-3 (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 pi-systems which drive electrostatic contributions, and lower energy electron poor pi-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 pi-bonding interactions dominate in all cases, while delta-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.