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 pi-acceptor ligand, in which the ligand sigma donates to the metal, which pi 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 pi* 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) 5f(1) ion that is pi back-bonded to the poor pi-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.