Back bonding without σ-bonding: a unique π-complex of dinitrogen with uranium

The synthesis, molecular structure, spectroscopic properties and bonding in a complex of dinitrogen with trivalent uranium are described. The molecule has a side-on U–N2–U core which is supported by U→N2 π donation with no significant N2→U bonding. Steric compression between the spectator triamidoamine ligands prevents the uranium atoms from approaching the N2 ligand at the optimum distance for overlap, and thus the N–N distance is not displaced significantly from that in gaseous dinitrogen.