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 where 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 organo-azide produces a uranium(V)–bis(imido)–dinitrogen complex, stabilised by a lithium counter-ion. This complex, which has been isolated in 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 rendering the uranium ion unusually electron-rich. This electron-poor back-bonding could have implications for the field of dinitrogen activation.