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.