Spontaneous Partitioning of Californium from Curium: Curious Cases from the Crystallization of Curium Coordination Complexes

The reaction of ²⁴⁸CmCl₃ with excess 2,6-pyridinedicarboxylic acid (DPA) under mild solvothermal conditions results in crystallization of the tris-chelate complex Cm­(HDPA)₃·H₂O. Approximately half of the curium remains in solution at the end of this process, and evaporation of the mother liquor results in crystallization of the bis-chelate complex [Cm­(HDPA)­(H₂DPA)­(H₂O)₂Cl]­Cl·2H₂O. ²⁴⁸Cm is the daughter of the α decay of ²⁵²Cf and is extracted in high purity from this parent. However, trace amounts of ^249,250,251Cf are still present in all samples of ²⁴⁸Cm. During the crystallization of Cm­(HDPA)₃·H₂O and [Cm­(HDPA)­(H₂DPA)­(H₂O)₂Cl]­Cl·2H₂O, californium­(III) spontaneously separates itself from the curium complexes and is found doped within crystals of DPA in the form of Cf­(HDPA)₃. These results add to the growing body of evidence that the chemistry of californium is fundamentally different from that of earlier actinides

Uncovering the Origin of Divergence in the CsM(CrO4)2 (M = La, Pr, Nd, Sm, Eu; Am) Family through Examination of the Chemical Bonding in a Molecular Cluster and by Band Structure Analysis

A series of f-block chromates, CsM­(CrO₄)₂ (M = La, Pr, Nd, Sm, Eu; Am), were prepared revealing notable differences between the Am^III derivatives and their lanthanide analogs. While all compounds form similar layered structures, the americium compound exhibits polymorphism and adopts both a structure isomorphous with the early lanthanides as well as one that possesses lower symmetry. Both polymorphs are dark red and possess band gaps that are smaller than the Ln^III compounds. In order to probe the origin of these differences, the electronic structure of α-CsSm­(CrO₄)₂, α-CsEu­(CrO₄)₂, and α-CsAm­(CrO₄)₂ were studied using both a molecular cluster approach featuring hybrid density functional theory and QTAIM analysis and by the periodic LDA+GA and LDA+DMFT methods. Notably, the covalent contributions to bonding by the f orbitals were found to be more than twice as large in the Am^III chromate than in the Sm^III and Eu^III compounds, and even larger in magnitude than the Am-5f spin–orbit splitting in this system. Our analysis indicates also that the Am–O covalency in α-CsAm­(CrO₄)₂ is driven by the degeneracy of the 5f and 2p orbitals, and not by orbital overlap

Emergence of californium as the second transitional element in the actinide series

vi+126hlm;14x21c

Monomers, Dimers, and Helices: Complexities of Cerium and Plutonium Phenanthrolinecarboxylates

The reaction of Ce^III or Pu^III with 1,10-phenanthroline-2,9-dicarboxylic acid (PDAH₂) results in the formation of new f-element coordination complexes. In the case of cerium, Ce­(PDA)­(H₂O)₂Cl·H₂O (<b>1</b>) or [Ce­(PDAH)­(PDA)]₂[Ce­(PDAH)­(PDA)] (<b>2</b>) was isolated depending on the Ce/ligand ratio in the reaction. The structure of <b>2</b> is composed of two distinct substructures that are constructed from the same monomer. This monomer is composed of a Ce^III cation bound by one PDA^2– dianionic ligand and one PDAH^– monoanionic ligand, both of which are tetradentate. Bridging by the carboxylate moieties leads to either [Ce­(PDAH)­(PDA)]₂ dimers or [Ce­(PDAH)­(PDA)]_1∞ helical chains. For plutonium, Pu­(PDA)₂ (<b>3</b>) was the only product isolated regardless of the Pu/ligand ratio employed in the reaction. During the reaction of plutonium with PDAH₂, Pu^III is oxidized to Pu^IV, generating <b>3</b>. This assignment is consistent with structural metrics and the optical absorption spectrum. Ambiguity in the assignment of the oxidation state of cerium in <b>1</b> and <b>2</b> from UV–vis–near-IR spectra invoked the use of Ce L_3,2-edge X-ray absorption near-edge spectroscopy, magnetic susceptibility, and heat capacity measurements. These experiments support the assignment of Ce^III in both compounds. The bond distances and coordination numbers are also consistent with these assignments. <b>3</b> contains 8-coordinate Pu^IV, whereas the cerium centers in <b>1</b> and <b>2</b> are 9- and/or 10-coordinate, which correlates with the increased size of Ce^III versus Pu^IV. Taken together, these data provide an example of a system where the differences in the redox behavior between these f elements creates more complex chemistry with cerium than with plutonium

Electronic Structure and Properties of Berkelium Iodates

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 Bk^III cations that are bridged by iodate anions to yield layers that are isomorphous with those found for Am^III, Cf^III, 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 Zr^IV cation. Bk^III–O and Bk^IV–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 Bk^III in these crystals. In addition to luminescence from Bk^III 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 (<i>t</i>_1/2 = 320 d) causes oxidation of Bk^III and only Bk^IV is present after a few days with concomitant loss of both the Bk^III 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 <i>ab initio</i> 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 Bk^IV that does not strictly adhere to Russel–Saunders coupling and Hund’s Rule even though it possesses a half-filled 5<i>f</i> ⁷ shell. Multiple factors contribute to the asymmetry that include 5<i>f</i> 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 Bk^IV–O bonds that distributes the 5<i>f</i> electrons onto the ligands. These factors are absent or diminished in other <i>f</i>⁷ ions such as Gd^III or Cm^III