Berkelium is element 97 in the periodic table. Since it was discovered in 1949, its properties have been largely unexplored. This actinide element has no known scientific, medical or industrial applications. It does not occur naturally on Earth. It is very expensive to synthesize and only a few milligrams of berkelium are available. It is also highly radioactive. Its most common isotope is Bk-249 with a half-life of three hundred and thirty-three days. It decays into californium which results in a build-up of electric charges in samples. Research suggests that berkelium’s bonds are mainly ionic and similar to lanthanides. This means that it is usually overlooked in favor of more readily available elements.
A new berkelium complex has shown that highly polarized ligands can be used to target heavier actinides. This suggests that it might be useful for selective recycling of radioactive elements. A team at Florida State University (FSU)has been investigating the sixth berkelium complex ever characterized. The situation may be more complex than originally thought. Thomas Albrect-Schönzart is the Gregory R. Choppin Professor at FSU and the leader of the berkelium research. His team bound Bk(III) to 4’-(4-nitrophenyl)-2,2:6’,2”-terpyridine, a ligand with a large dipole.
The team hoped that this would polarize the berkelium electrons when the metal-ligand bonds were formed. The team also formed a metal-ligand complex with cerium, berkelium’s closest electro-chemical analogue. This was done to compare any effects through structural analysis, spectroscopy and electrochemical analysis.
The FSU team discovered that the polarization of the ligand shortened the metal-ligand bonds in the same plane more than was expected. This influenced the electron density. The team also found signs that there had been a reduction in inter-electron repulsion. 6-p orbitals are hybridized with berkelium and 5p orbitals, to a lesser extent, with cerium. Albrecht–Schönzart said, “The 6p orbitals are normally thought of as core orbitals not involved in bonding. But here they are hybridizing with the ligand orbitals, and this even creates a covalent bond with the water molecule trans to the ligand. The effects of the polarization were much larger than anyone anticipated.”
In addition, electron paramagnetic resonance (EPR) spectra displayed a rhombus-shaped signal when the cerium complex was investigated. This suggests that the electronic environment around the center of the complex was highly anisotropic.
This unexpected complexity of the heavier actinides suggests that the properties of berkelium may be far more interesting than earlier research suggested. The custom-designed complex at FSU also acts as a proof-of-concept for creating highly polarized ligands that can achieve specific bond strengths with target molecules. This property would allow scientists to custom-design ligands that target specific metals during recycling of radioactive materials. This would allow for the selective extraction of specific elements.
Conrad Goodwin is an actinide chemist at the University of Manchester in the U.K. He says that the new FSU papers is an important example that highlights the complexity of the heavier actinides. He went on to say, “It’s difficult to overstate how painful each advancement in this area – particularly with berkelium – can be both from a logistical standpoint, and also just material availability. This is a fascinating contribution to the fundamental coordination chemistry of elements beyond uranium.”