Back-bonding between an electron-poor, high-oxidation-state metal and poor π-acceptor ligand in a uranium(v)–dinitrogen complex

A fundamental bonding model in coordination and organometallic chemistry is the synergic, donor–acceptor interaction between a metal and a neutral π-acceptor ligand, in which the ligand σ donates to the metal, which π back-bonds to the ligand. This interaction typically involves a metal with an electron-rich, mid-, low- or even negative oxidation state and a ligand with a π* orbital. Here, we report that treatment of a uranium–carbene complex with an organoazide produces a uranium(v)–bis(imido)–dinitrogen complex, stabilized by a lithium counterion. This complex, which was isolated in a crystalline form, involves an electron-poor, high-oxidation-state uranium(v) 5f1 ion that is π back-bonded to the poor π-acceptor ligand dinitrogen. We propose that this is made possible by a combination of cooperative heterobimetallic uranium–lithium effects and the presence of suitable ancillary ligands that render the uranium ion unusually electron rich. This electron-poor back-bonding could have implications for the field of dinitrogen activation.