Introduction
In the periodic table, berkelium is at a transition point between the
lighter actinides exhibiting multiple oxidation states and the heavier
actinides behaving more like the lanthanides that are dominated by the
trivalent state.1 Berkelium exhibits both the
trivalent and tetravalent oxidation states that are stable in solution
and solid state. Berkelium(IV) has special stability as a consequence of
its half-filled shell 5f7 electronic configuration.
Because of the higher charge of tetravalent berkelium and its affinity
for strong complexation by hard oxygen-donor ligands such as carbonate
ions, Bk(IV) is more stable than Bk(III) in strongly complexing,
concentrated, basic carbonate solutions. In fact, green Bk(III)
auto-oxidizes in air in carbonate solutions to form yellow Bk(IV)
complexes.2
There is an increased interest in berkelium chemistry because of its
unique position in the periodic table, its importance as a target
material for production of super-heavy elements, and the recent
availability of multi-milligram quantities of the relatively long-lived
isotope 249Bk (t½ = 330
d).1,3 In contrast, only a few reports of theoretical
studies of Bk(III)4–6 and
Bk(IV)3,5,6 complexes have been reported.
Experimental, spectroscopic, and electrochemical studies of berkelium in
concentrated basic aqueous carbonate solutions were published in the
1990’s that demonstrated that carbonate ions stabilize
Bk(IV).2,7 The present study aims to give further
theoretical insight into the electronic structure and bonding of Bk(IV)
in carbonate and carbonate-hydroxide environments.