More information: Dominic R. Russo et al, Berkelium–carbon bonding in a tetravalent berkelocene, Science (2025). DOI: 10.1126 ...

Normally, a carbon atom shares all four of its valence electrons to create four bonds with other atoms (tetravalent carbon).

The calculations showed 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.