Mounting evidence suggests that actinide bonds have notable covalent character, making their chemistry more akin to transition metals than lanthanides. That covalency can be directly measured using pulsed electron paramagnetic resonance spectroscopy, yielding some surprising results, reports a team from the University of Manchester (Nat. Chem. 2016, DOI: 10.1038/nchem.2692). So-called superhyperfine coupling of a metal’s unpaired electrons with ligand nuclei that have non-zero spin can be used to assess covalency, but signals couldn’t be resolved by older EPR techniques. Using newer pulsed methods, Floriana Tuna, Eric J. L. McInnes, David P. Mills, and coworkers studied thorium(III) and uranium(III) organometallic complexes with ligands derived from the cyclopentadienyl anion (Cp). Computational analysis of the Th and U complexes suggested that they had very similar, weakly covalent bonds. But the EPR measurements indicate that the complexes differ, with much more covalency in U-Cp bonds than in Th-Cp bonds. Unexpectedly, the Th complex more closely resembles the analogous lanthanide ytterbium complex from the opposite end of the periodic table. “Such results highlight the need for new experimental data on systematic families of well-defined complexes,” the authors say.