A californium complex has provided the first crystal structure featuring a californium-carbon bond, helping probe bonding trends in the nether regions of the periodic table (Nature 2021, DOI: 10.1038/s41586-021-04027-8). Californium is the heaviest element available in milligram quantities. Its scarcity and radioactivity pose huge experimental challenges. “We’re really pushing the limits of the smallest scale at which you can do classical synthetic chemistry,” says Andrew Gaunt, a researcher at Los Alamos National Laboratory and part of the team that created the complex. The researchers combined 2 mg of 249Cf3+ with potassium tetramethylcyclopentadienide in diethyl ether, forming crystals of a dark-orange metallocene complex (shown). Metallocenes are compounds in which aromatic ligands sandwich a metal ion. Most californium complexes are light green; the metallocene’s unusual color is likely due to an electron being excited from cyclopentadienyl to californium. X-ray crystallography revealed that the two cyclopentadienyl ligands were separated by a 131° angle, making this the first structure of a bent metallocene involving an element heavier than uranium. Electronic structure calculations showed that the californium-carbon bond is largely ionic in nature. The finding should help researchers understand trends in bonding across the actinide series, such as the degree of covalent bonding with ligands—an important basis for developing more effective ways to separate these elements from spent nuclear fuel. “This concept of covalency and actinide separations is still a very hot topic,” Gaunt says.