A berkelium(III) mellitate, Bk2[C6(CO2)6](H2O)8·2H2O, was synthesized and rapidly crystallized by reacting mellitic acid, C6(CO2H)6, and BkBr3·nH2O in an aqueous medium. Single crystal X-ray diffraction shows that the compound crystallizes as a three-dimensional framework isostructural with Pu(III), Am(III), and Cm(III) mellitates. UV-vis-NIR spectroscopic studies as a function of pressure were performed using a diamond anvil cell and show that the 5f → 5f transitions of Bk3+ display enhanced hypsochromic shifting when compared to other An(III) mellitates.

Despite the significant impact of radiation-induced redox reactions on the accessibility and lifetimes of actinide oxidation states, fundamental knowledge of aqueous actinide metal ion radiation chemistry is limited, especially for the late actinides. A quantitative understanding of these intrinsic radiation-induced processes is essential for investigating the fundamental properties of these actinides. We present here a picosecond electron pulse reaction kinetics study into the radiation-induced redox chemistry of trivalent berkelium (Bk(III)) and californium (Cf(III)) ions in acidic aqueous solutions at ambient temperature. New and first-of-a-kind, second-order rate coefficients are reported for the transient radical-induced reduction of Bk(III) and Cf(III) by the hydrated electron (eaq-) and hydrogen atom (H•), demonstrating a significant reactivity (up to 1011 M-1 s-1) indicative of a preference of these metals to adopt divalent states. Additionally, we report the first-ever second-order rate coefficients for the transient radical-induced oxidation of these elements by a reaction with hydroxyl (•OH) and nitrate (NO3•) radicals, which also exhibited fast reactivity (ca. 108 M-1 s-1). Transient Cf(II), Cf(IV), and Bk(IV) absorption spectra are also reported. Overall, the presented data highlight the existence of rich, complex, intrinsic late actinide radiation-induced redox chemistry that has the potential to influence the findings of other areas of actinide science.

The reaction of 249Bk(OH)4 with iodate under hydrothermal conditions results in the formation of Bk(IO3)3 as the major product with trace amounts of Bk(IO3)4 also crystallizing from the reaction mixture. The structure of Bk(IO3)3 consists of nine-coordinate BkIII cations that are bridged by iodate anions to yield layers that are isomorphous with those found for AmIII, CfIII, and with lanthanides that possess similar ionic radii. Bk(IO3)4 was expected to adopt the same structure as M(IO3)4 (M = Ce, Np, Pu), but instead parallels the structural chemistry of the smaller ZrIV cation. BkIII-O and BkIV-O bond lengths are shorter than anticipated and provide further support for a postcurium break in the actinide series. Photoluminescence and absorption spectra collected from single crystals of Bk(IO3)4 show evidence for doping with BkIII in these crystals. In addition to luminescence from BkIII in the Bk(IO3)4 crystals, a broad-band absorption feature is initially present that is similar to features observed in systems with intervalence charge transfer. However, the high-specific activity of 249Bk (t1/2 = 320 d) causes oxidation of BkIII and only BkIV is present after a few days with concomitant loss of both the BkIII luminescence and the broadband feature. The electronic structure of Bk(IO3)3 and Bk(IO3)4 were examined using a range of computational methods that include density functional theory both on clusters and on periodic structures, relativistic ab initio wave function calculations that incorporate spin-orbit coupling (CASSCF), and by a full-model Hamiltonian with spin-orbit coupling and Slater-Condon parameters (CONDON). Some of these methods provide evidence for an asymmetric ground state present in BkIV that does not strictly adhere to Russel-Saunders coupling and Hund's Rule even though it possesses a half-filled 5f 7 shell. Multiple factors contribute to the asymmetry that include 5f electrons being present in microstates that are not solely spin up, spin-orbit coupling induced mixing of low-lying excited states with the ground state, and covalency in the BkIV-O bonds that distributes the 5f electrons onto the ligands. These factors are absent or diminished in other f7 ions such as GdIII or CmIII.

Berkelium is positioned at a crucial location in the actinide series between the inherently stable half-filled 5f(7) configuration of curium and the abrupt transition in chemical behavior created by the onset of a metastable divalent state that starts at californium. However, the mere 320-day half-life of berkelium's only available isotope, (249)Bk, has hindered in-depth studies of the element's coordination chemistry. Herein, we report the synthesis and detailed solid-state and solution-phase characterization of a berkelium coordination complex, Bk(III)tris(dipicolinate), as well as a chemically distinct Bk(III) borate material for comparison. We demonstrate that berkelium's complexation is analogous to that of californium. However, from a range of spectroscopic techniques and quantum mechanical calculations, it is clear that spin-orbit coupling contributes significantly to berkelium's multiconfigurational ground state.