Berkelocene puts exotic berkelium in a sandwich

It is a sandwich with the rarest of fillings. Researchers have unveiled berkelocene, a compound containing the synthetic element berkelium nestled neatly between two ring-shaped molecules (Science 2025, DOI: 10.1126/science.adr3346).

Berkelocene pushes the frontiers of organometallic chemistry deeper into the lower reaches of the periodic table and could help solve puzzles about the nature of chemical bonding in the heaviest elements. “It’s an amazing bit of experimental work,” says Conrad Goodwin, who studies lanthanide and actinide chemistry at the University of Manchester and was not involved in the research. “I always thought that you'd be able to make something like this, and I’m so glad that they did it.”

Berkelocene traces its family tree back to that 1950s icon of organometallic chemistry, ferrocene, which sandwiches iron between two cyclopentadienyl rings. In the late 1960s, researchers found that the actinide element uranium could be sandwiched in a similar way, albeit with slightly larger slices of bread made from cyclooctatetraene (COT), which is a better fit for the heavy metal’s 5f electron orbitals.

Sandwiches of COT with other actinides soon followed, the heaviest being plutonium. “Those molecules have been foundational to our current understanding of actinide bonding,” says Stefan Minasian of Lawrence Berkeley National Laboratory (LBNL), part of the team behind the new berkelium complex.

To push even further along the actinide series, the researchers studied various analogs of COT to identify one that would offer the best chances of forming a decent amount of berkelocene crystals. Then they honed their experimental procedures using cerium as a surrogate.

The berkelium itself came from the Oak Ridge National Laboratory High Flux Isotope Reactor, which makes about 13 mg of ²⁴⁹Bk for researchers every couple of years. A team led by Thomas Albrecht at the Colorado School of Mines had some berkelium left over from a recent allotment and donated roughly 1 mg of its share to the LBNL group.

Berkelium-249 has a half-life of 330 days and decays to californium-249, a dangerous daughter that emits intense, high-energy γ radiation. These ferociously radioactive materials are also sensitive to air and moisture, so chemists have to handle the tiny amounts of material in a glove box.

“This is a very precious, priceless resource. And we only have one shot, maybe two, if we're lucky, to make it work,” Minasian says. After months of preparation, the berkelocene synthesis was carried out by PhD student Dominic Russo, who had worked on the COT derivative (hdtCOT) deemed most suitable.

The procedure used about 0.3 mg of berkelium in the form of BkCl₃, which also contained 0.2 mg of californium. Adding the hdtCOT and an oxidizing agent brought metal and ligand together, ultimately forming indigo crystals of berkelocene. “My group tried to do this for 4 years, unsuccessfully,” Albrecht says. “I think it's world-class work.”

Berkelocene is likely to be the last of the actinide-COT sandwiches, he adds, because heavier elements either are too scarce or struggle to reach the required oxidation state. “In terms of a molecule that someone can see in a vial, that's the end,” Albrecht says.

Berkelocene shows some unique bonding characteristics. In the analogous cerium compound, for example, each cerium atom is a strange hybrid of mostly Ce³⁺ with a little Ce⁴⁺. But in berkelocene, the Bk⁴⁺ state is dominant, perhaps because of better overlap between the metal and ligand orbitals. “This nicely shows that the chemistry of the trans-plutonium actinides is not well represented by the lanthanides or by the early actinides,” Minasian says.

Only a handful of other organometallic compounds have been made with elements heavier than plutonium, so berkelocene adds a vital data point that could improve theoretical calculations about the chemistry of highly radioactive trace elements found in nuclear waste. “That helps us with predicting things that are too dangerous or too risky for us to work with,” LBNL team member Polly Arnold says.