Uranium nitride triple bond is surprisingly covalent

An unusual uranium nitride complex contains a highly covalent triple bond that pushes the boundaries of actinide bonding (Nat. Commun. 2021, DOI: 10.1038/s41467-021-25863-2). Covalency describes the degree of electron sharing between two atoms. In general, the earlier actinides tend to engage in ionic or polarized covalent bonding. This bonding character falls between the lanthanides, which prefer ionic bonding, and transition-metal complexes, which can have much more covalent bonds. But in 2013, a team led by Stephen T. Liddle, now at the University of Manchester, created a terminal uranium(VI)-nitride complex (shown) that was computationally predicted to have a remarkably covalent U≡N bond. Liddle’s team has now experimentally confirmed this with ¹⁵N nuclear magnetic resonance spectroscopy, after synthesizing a version of the complex containing the isotope. NMR data showed that the U≡N bond is even more covalent than analogous transition metal–nitride complexes. “We were really surprised,” Liddle says. “We have to redraw the map of just how covalent the actinides can be.” A deeper understanding of actinide bonding could help design better ligands for separating elements in nuclear waste, he adds. The team also created computational models for a range of metal-nitrogen bonds and compared them with experimental ¹⁵N NMR data for 17 transition-metal and actinide complexes, including the uranium nitride. The clear trend between NMR data and bonding means ¹⁵N NMR could be a convenient tool to determine the covalency of other metal-nitrogen bonds, Liddle says.