Compounds of berkelium

Berkelium forms a number of chemical compounds, where it normally exists in an oxidation state of +3 or +4, and behaves similarly to its lanthanide analogue, terbium. Like all actinides, berkelium easily dissolves in various aqueous inorganic acids, liberating gaseous hydrogen and converting into the trivalent oxidation state. This trivalent state is the most stable, especially in aqueous solutions, but tetravalent berkelium compounds are also known. The existence of divalent berkelium salts is uncertain and has only been reported in mixed lanthanum chloride-strontium chloride melts. Aqueous solutions of Bk3+ ions are green in most acids. The color of the Bk4+ ions is yellow in hydrochloric acid and orange-yellow in sulfuric acid. Berkelium does not react rapidly with oxygen at room temperature, possibly due to the formation of a protective oxide surface layer; however, it reacts with molten metals, hydrogen, halogens, chalcogens and pnictogens to form various binary compounds. Berkelium can also form several organometallic compounds. Two oxides of berkelium are known, with berkelium in the +3 (Bk2O3) and +4 (BkO2) oxidation states. Berkelium(IV) oxide is a brown solid that crystallizes in a cubic (fluorite) crystal structure with the space group Fm3m and the coordination numbers of Bk and O. The lattice parameter is 533.4 ± 0.5 pm. Berkelium(III) oxide, a yellow-green solid, is formed from BkO2 by reduction with hydrogen: The compound has a melting point of 1920 °C, body-centered cubic crystal lattice and a lattice constant a = 1088.0 ± 0.5 pm. Upon heating to 1200 °C, the cubic Bk2O3 transforms to a monoclinic structure, which further converts to a hexagonal phase at 1750 °C; the latter transition is reversible. Such three-phase behavior is typical for the actinide sesquioxides. A divalent oxide BkO has been reported as a brittle gray solid with a face centered cubic (fcc) structure and a lattice constant a = 496.4 pm, but its exact chemical composition is uncertain. In halides, berkelium assumes the oxidation states +3 and +4. The +3 state is most stable, especially in solutions, and the tetravalent halides BkF4 and Cs2BkCl6 are only known in the solid phase. The coordination of the berkelium atom in its trivalent fluoride and chloride is tricapped trigonal prismatic, with a coordination number of 9. In the trivalent bromide, it is bicapped trigonal prismatic (coordination 8) or octahedral (coordination 6), and in the iodide it is octahedral. Berkelium(IV) fluoride (BkF4) is a yellow-green ionic solid which crystallizes in the monoclinic crystal system (Pearson symbol mS60, space group C2/c No. 15, lattice constants a = 1247 pm, b = 1058 pm, c = 817 pm) and is isotypic with uranium tetrafluoride or zirconium(IV) fluoride. Berkelium(III) fluoride (BkF3) is also a yellow-green solid, but it has two crystalline structures. The most stable phase at low temperatures has an orthorhombic symmetry, isotypic with yttrium(III) fluoride (Pearson symbol oP16, space group Pnma, No. 62, a = 670 pm, b = 709 pm, c = 441 pm). Upon heating to 350 to 600 °C, it transforms to a trigonal structure found in lanthanum(III) fluoride (Pearson symbol hP24, space group P3c1, No. 165, a = 697 pm, c = 714 pm).