Berkelocene: Chemists Create Berkelium-Containing Organometallic Molecule

Organometallic molecules consist of a metal ion surrounded by a carbon-based framework. They are relatively common for early actinide elements like uranium but are scarcely known for later actinides. Scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) have now prepared an organometallic complex from 0.3 milligrams of berkelium-249.

Berkelium, one of 15 actinides in the periodic table’s f-block, was discovered by the pioneering nuclear chemist Glenn Seaborg in 1949.

But this heavy element is not easy to study because it is highly radioactive; and only very minute amounts are produced globally every year.

“This is the first time that evidence for the formation of a chemical bond between berkelium and carbon has been obtained,” said Dr. Stefan Minasian, a scientist at Berkeley Lab.

“The discovery provides new understanding of how berkelium and other actinides behave relative to their peers in the periodic table.”

“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,” added Professor Polly Arnold, a chemist at the University of California, Berkeley, and director of Berkeley Lab’s Chemical Sciences Division.

At Berkeley Lab’s Heavy Element Research Laboratory, the researchers designed new gloveboxes enabling air-free syntheses with highly radioactive isotopes.

Then, with just 0.3 milligram of berkelium-249, they conducted single-crystal X-ray diffraction experiments.

Their results showed a symmetrical structure with the berkelium atom sandwiched between two 8-membered carbon rings.

The scientists named the new molecule berkelocene, because its structure is analogous to a uranium organometallic complex called uranocene.

In an unexpected finding, electronic structure calculations revealed that 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,” Dr. Minasian said.

“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,” Professor Arnold added.

“More accurate models showing how actinide behavior changes across the periodic table are needed to solve problems related to long-term nuclear waste storage and remediation.”

“This clearer portrait of later actinides like berkelium provides a new lens into the behavior of these fascinating elements,” said Dr. Rebecca Abergel, a researcher at Berkeley Lab and the University of California, Berkeley.

A paper describing this research was published in the journal Science.

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Dominic R. Russo et al. 2025. Berkelium-carbon bonding in a tetravalent berkelocene. Science 387 (6737): 974-978; doi: 10.1126/science.adr3346