Berkeley Lab’s 48-Hour race against time with new molecule berkelocene: A step toward safer nuclear waste management?

With just two days to work before their microscopic sample of ultra-rare berkelium degraded, scientists at Lawrence Berkeley National Laboratory created “berkelocene”—a brand-new molecule that could offer insights for safely managing nuclear waste.

While berkelocene itself won’t be directly used in nuclear waste management, the research holds clues for safer approaches. Nuclear waste contains problematic “minor actinides” like americium and curium; studying related elements such as berkelium helps understand this group, which contributes significantly to long-term radioactivity and heat generation. Scientists need accurate predictive models for these elements to design safe storage strategies and effective treatment processes.

Such models rely partly on understanding how these elements might interact with container materials, surrounding rock, or groundwater, and whether they can be selectively removed from waste. Current theoretical models have limitations, especially for elements heavier than plutonium, because they rely on experimental data that remains scarce for these rare, difficult-to-handle elements. The berkelocene research provides experimental data to test and refine these models. For instance, the study revealed that berkelium could be stabilized in the +4 oxidation state with significant covalent bonding, a finding not necessarily predicted by simpler models. Berkelium-249 has a half-life of only 330 days and decays into californium-249.

This is the first time that evidence for the formation of a chemical bond between berkelium and carbon has been obtained.

This data from the new research could allow scientists to improve their computational models for predicting how actinides behave, potentially yielding more reliable predictions about actinide behavior in waste repositories and informing the design of new chemical processes for separating specific actinides.

Racing against decay

Working with just 0.3 milligrams of berkelium-249 (²⁴⁹Bk)—an isotope so rare that less than one gram has been produced in the U.S. since 1967 (primarily at Oak Ridge National Laboratory)—the Berkeley Lab team operated under extraordinary constraints. Because ²⁴⁹Bk has a half-life of only 330 days and decays into californium-249, time was critical. The scientists used specialized chemical glove boxes with inert atmospheres to shield the highly reactive material from oxygen and moisture.

The team first prepared anhydrous berkelium chloride, then treated it with modified cyclooctatetraene rings. When the resulting solution turned a deep indigo—rather than an orange-red indicating failure—the researchers knew they had successfully synthesized their target molecule, berkelocene. The research, led by principal investigators Stefan Minasian and Polly Arnold, is featured in Science.

Structurally, berkelocene resembles uranocene, a similar uranium-based sandwich compound discovered in the late 1960s. What makes berkelocene particularly noteworthy is its stabilization of berkelium in the tetravalent (Bk⁴⁺) oxidation state. This state is difficult to achieve for many transplutonium elements but is more accessible for berkelium due to the relative stability of its half-filled 5f⁷ electronic configuration—a quantum mechanical arrangement lending unique chemical properties.

Single-crystal X-ray diffraction confirmed the sandwich structure, showing the Bk⁴⁺ ion positioned between two substituted cyclooctatetraene ligands. Advanced spectroscopic analysis and theoretical calculations further verified the existence of covalent bonds involving the f orbitals of the berkelium metal and the carbon ligands—providing a level of detail previously unavailable for organometallic complexes of elements this heavy.

Dealing with significant hurdles

The team overcame significant scientific and technical challenges, including the scarcity and high radioactivity of ²⁴⁹Bk. Synthetic protocols were developed for ultrasmall scale reactions and optimized using cerium, a nonradioactive element with similar properties, as a surrogate before working with the actual berkelium.

This achievement is significant as it demonstrates that stable berkelium-carbon bonds can be formed in organometallic complexes and that such compounds involving rare, radioactive isotopes can be isolated and fully characterized starting from sub-milligram quantities.