New compounds on the Bk menu

For f-block elements like uranium, there is stabilizing coordination chemistry that can be used to demonstrate how the element reacts and forms bonds, but some of the more exotic elements in the f-block have not been isolated within compounds in this way. Complexes of plutonium, americium, curium and californium have provided spectroscopic insights into the interesting electronic structure of the actinides, but the gap between them — berkelium — leaves questions to be answered about the factors controlling their properties. Now, a group of researchers led by Jenifer Braley and Thomas Albrecht-Schmitt at the Colorado School of Mines and Florida State University, respectively, have isolated and characterized the first complexes of berkelium(III) (Science 353, aaf3762; 2016).

Credit: AAAS

The only available isotope of berkelium is ²⁴⁹Bk, which has a half-life of only 320 days. However, Braley, Albrecht-Schmitt and co-workers developed procedures for its rapid precipitation and metallation with dipicolinic acid. The resulting complexes were characterized by X-ray diffraction, revealing D₃ symmetric nine-coordinate Bk(III) complexes with monoprotonated dipicolinate ligands. The compounds have structural characteristics that are, unsurprisingly, in between those of californium and curium, however, spectroscopic measurements revealed that their electronic structures are unusual. In particular, a band in the absorption and excitation spectra revealed that the 6d orbitals of Bk(III) are lower than those in other actinides with the same ligands. This is consistent with 5f–6d coupling that would be expected based on the D₃ symmetry of the complexes and contributes to the unusually high degree of covalency in the Bk–ligand bonds. These characteristics make berkelium unique among actinides.

Magnetic and theoretical analyses were carried out and comparisons between berkelium and other superheavy elements made. Despite similar electronic configurations to terbium, the electronic properties of these complexes are distinct, with spin–orbit coupling in Bk(III) dominating the ligand-field effects. The ligand-field effects of the berkelium compounds studied were observed to be similar to those of curium, but although spin–orbit coupling also dictates the electronic structure of Cf(III), ligand-field splitting in Cf(III) compounds is much greater than in those of Bk(III). This work emphasizes that spin–orbit coupling is a dominating factor in determining the ground-state structures of late actinide complexes, which may well inform the ways in which other elements from this part of the periodic table are not only characterized, but perhaps stabilized for similar studies.