Scientists Find New Heavy Metal Molecule ‘Berkelocene’

In a significant advancement for heavy-element chemistry, researchers at Lawrence Berkeley National Laboratory have successfully created and characterized “berkelocene,” the first organometallic molecule containing berkelium-carbon bonds. The achievement, published last week in the journal Science, challenges traditional understanding of how heavy radioactive elements interact with carbon-based structures.

Working with just 0.3 milligrams of radioactive berkelium-249, the team crafted a molecule where the berkelium atom sits “sandwiched” between two carbon rings, similar to how a hamburger patty sits between two buns. This configuration allowed scientists to observe direct chemical bonding between berkelium and carbon atoms for the first time.

“This is the first time that evidence for the formation of a chemical bond between berkelium and carbon has been obtained,” said Stefan Minasian, a scientist in Berkeley Lab’s Chemical Sciences Division and one of the study’s four co-corresponding authors. “The discovery provides new understanding of how berkelium and other actinides behave relative to their peers in the periodic table.”

Berkelium, with atomic number 97, belongs to the actinide series—15 radioactive elements in the periodic table’s f-block. The element has historical significance to Berkeley Lab, having been discovered there in 1949 by Glenn Seaborg, who later won the 1951 Nobel Prize in Chemistry for his work on transuranium elements.

Creating the berkelocene molecule presented multiple challenges. Berkelium is highly radioactive, extremely rare, and only minute quantities are produced globally each year. Additionally, organometallic molecules are notoriously unstable in air.

“Only a few facilities around the world can protect both the compound and the worker while managing the combined hazards of a highly radioactive material that reacts vigorously with the oxygen and moisture in air,” explained Polly Arnold, director of Berkeley Lab’s Chemical Sciences Division and another co-corresponding author.

To overcome these obstacles, the team designed custom gloveboxes at Berkeley Lab’s Heavy Element Research Laboratory that enabled air-free synthesis with highly radioactive isotopes. The isotope used in the experiment was distributed from the National Isotope Development Center, managed by the Department of Energy Isotope Program at Oak Ridge National Laboratory.

The research revealed an unexpected finding—the berkelium atom at the center of the berkelocene structure has a tetravalent oxidation state (positive charge of +4), which is stabilized by the berkelium–carbon bonds.

“Traditional understanding of the periodic table suggests that berkelium would behave like the lanthanide terbium,” said Minasian. “But the berkelium ion is much happier in the +4 oxidation state than the other f-block ions we expected it to be most like,” Arnold added.

Rebecca Abergel, who leads the Heavy Element Chemistry Group at Berkeley Lab and served as another co-corresponding author, emphasized the significance of their findings: “This clearer portrait of later actinides like berkelium provides a new lens into the behavior of these fascinating elements.”

The research team used single-crystal X-ray diffraction experiments to determine the structure of berkelocene, revealing a symmetrical arrangement with the berkelium atom sandwiched between two 8-membered carbon rings. The molecule’s structure resembles “uranocene,” a uranium organometallic complex discovered in the late 1960s by UC Berkeley chemists Andrew Streitwieser and Kenneth Raymond.

Electronic structure calculations performed by co-corresponding author Jochen Autschbach at the University at Buffalo provided deeper insights into berkelium’s unique behavior, helping explain why it forms stable bonds with carbon in ways that differ from related elements.

The team believes their work could help develop more accurate models showing how actinide behavior changes across the periodic table—knowledge that may prove valuable for addressing challenges related to long-term nuclear waste storage and remediation.

Beyond its immediate scientific implications, the berkelocene discovery represents a technical achievement in handling extremely scarce and hazardous materials. The entire experimental process, from synthesis to characterization, had to be completed within about 48 hours to minimize complications from radioactive decay.

The researchers custom-designed their approach using cerium—a non-radioactive element with similar chemical properties—as a surrogate to ensure their procedures would work with the precious berkelium sample.

While many actinide organometallic compounds have been studied since they were first investigated during the Manhattan Project, compounds involving elements heavier than plutonium remain rare. The berkelocene discovery builds upon recent advances in transplutonium chemistry, including the first structural verification of americium-carbon and californium-carbon bonds reported in 2019 and 2021.

The work was supported by the Department of Energy’s Office of Science and provides new avenues for exploring the fundamental chemistry of heavy elements at the far reaches of the periodic table.