Since it was first synthesized in a post-WW2 American lab in 1949, berkelium has been a rebel of the periodic table, defying quantum mechanics and taking on an extra positive charge that its relatives would never.
Now, a team of scientists from berkelium’s alma mater, Lawrence Berkeley National Laboratory, has wrangled the elusive element into a rare partnership with carbon that will enable them to study it in more detail.
Thanks to challenges involved in producing and safely containing the heavy element, few chemists have had the privilege of dealing with berkelium. Just one gram of the stuff can cost a boggling US$27 million. For this experiment, just 0.3 milligrams of berkelium-249 was required.
Such heavy, radioactive chemicals are difficult to study on their own. In the form of an organometallic complex – with their high symmetries and multiple covalent bonds with carbon – it’s also much easier to probe the atom’s electronic structure.
However, the resulting molecular structure is so reactive with air that only a few laboratories in the world can protect both it and the person working on it.
The configuration of this molecule, ‘berkelocene’, is modelled on a similar structure called ferrocene, but instead of a charged iron filling, an ion of the radioactive element berkelium is sandwiched between two carbon rings to form an organometallic complex. In doing so, they hope to better understand this highly radioactive element, and perhaps its behavior in materials like nuclear waste.
Heavy element researchers have been keen to lock down the 15 radioactive elements in the periodic table’s actinide series using carbon-based cuffs ever since they trapped uranium in the far more thermodynamically stable form of uranocene.
Through the 60s and 70s, chemists proceeded to work their way through the list of potential actinocenes: by 1970, they had created thorocene from thorium, protactinocene from protactinium, neptunocene from neptunium, and plutonocene from plutonium.
And in recent years, chemists have even achieved organometallic complexes containing the heavier actinides americium and californium.
But berkelium, at a dizzy number 97 on the periodic table, has evaded its actinocene destiny until now.
“This is the first time that evidence for the formation of a chemical bond between berkelium and carbon has been obtained,” says Berkley Lab chemist Stefan Minasian. “The discovery provides new understanding of how berkelium and other actinides behave relative to their peers in the periodic table.”
By pinning down the berkelium atom, the team could test its electronic structure model using ultraviolet–visible–near-infrared spectroscopy.
“Traditional understanding of the periodic table suggests that berkelium would behave like the lanthanide terbium,” Minasian says. Yet, unlike the lanthanide analogues, the berkelium ion is happier in a ‘+4’ charged state, which suggests it is ionic bonds sticking the organometallic molecule together like two magnets, rather than the stronger glue of covalent bonds.
Single-crystal x-ray diffraction of the resulting organometallic molecule reveals the berkelium atom is held in place by two rings made up of carbon and hydrogen atoms, bonding with the carbon atoms.
The researchers hope that by understanding more about the heavier actinides’ behaviors, we can be prepared for problems arising from long-term nuclear waste storage and clean-up, as these unstable synthetic elements wend their way down the periodic table.
This research was published in Science.
