The hydration reaction is defined as the transfer of an ion or neutral chemical species from the gaseous phase into water, M n+ (g) → M n+ (aq). In this process, water mole- cules bind to metal ions through ion-dipole bonds of mainly electrostatic character. The hy- dration reaction is always strongly exothermic with increa sing heat of hydration with in- creasing charge density of the ion. The structures of the hydrated metal ions in aqueous solution display a variety of configurations depending on the size and electronic properties of the metal ion. The basic configurations of hydrated metal ions in aqueous solution are tetrahedral, octahedral, square antiprismatic, and tricapped trigonal prismatic. This paper gives an overview of the structures of hydrated metal ions in aqueous solution with special emphasis on those with a non-regular coordination figure. Metal ions without d-electrons in the valance shell form regular aqua complexes with a coordination figure, allowing a maxi- mum number of water molecules to be clustered around the metal ion. This number is de- pendent on the ratio of the metal ion radius to the atomic radius of oxygen in a coordinated water molecule (1.34 A). The lighter lanthanoid(III) ions have a regular tricapped trigonal prismatic configuration with the M-O distance to the capping water molecules somewhat longer than to the prismatic ones. However, with increasing atomic number of the lan- thanoid(III) ions, an increasing distortion of the capping water molecules is observed, result- ing in a partial loss of water molecules in the capping positions for the heaviest lanthanoids. Metal ions with d 4 and d 9 valance shell electron configuration, as chromium(II) and cop- per(II), respectively, have Jahn-Teller distorted aqua complexes. Metal ions with low charge and ability to form strong covalent bonds, as silver(I), mercury(II), palladium(II), and plat- inum(II), often display distorted coordination figures due to the second-order Jahn-Teller ef- fect. Metal ions with d 10 s 2 valence shell electron configuration may have a stereochemically active lone electron pair (hemi-directed complexes) or an inactive one (holo-directed). The hydrated tin(II), lead(II), and thallium(I) ions are hemi-directed in aqueous solution, while the hydrated bismuth(III) ion is holo-directed. The structures of the hydrated cationic oxo- metal ions are reported as well.

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Hydrated metal ions in aqueous solution: How regular are their structures?

Solution and solid-state structural chemistry of actinide hydrates and their hydrolysis and condensation products.

Actinide colloids and particles of environmental concern

The inner-sphere mechanisms of the disproportionation reactions of U(V), Np(V), and Pu(V) ions have been studied using a quantum mechanical approach. The U(V) disproportionation proceeds via the formation of a dimer (a cation−cation complex) followed by two successive protonations at the axial oxygens of the donor uranyl ion. Bond lengths and spin multiplicities indicate that electron transfer occurs after the first protonation. A solvent water molecule then breaks the complex into solvated U(OH)22+ and UO22+ ions. Pu(V) behaves similarly, but Np(V) appears not to follow this path. The observations from quantum modeling are consistent with existing experimental data on actinyl(V) disproportionation reactions.

A theoretical study of the inner-sphere disproportionation reaction mechanism of the pentavalent actinyl ions.

Analysis of X-ray absorption fine structure spectra of UO(2+x) for x = 0-0.20 (UO(2)--U(4)O(9)) reveals that the adventitious O atoms are incorporated as oxo groups with U--O distances of 1.74 A, most likely associated with U(VI), that occur in clusters so that the UO(2) fraction of the material largely remains intact. In addition to the formation of some additional longer U--O bonds, the U sublattice consists of an ordered portion that displays the original U--U distance and a spectroscopically silent, glassy part. This is very different from previous models derived from neutron diffraction that maintained long U--O distances and high U--O coordination numbers. UO(2+x) also differs from PuO(2+x) in its substantially shorter An-oxo distances and no sign of stable coordination with H(2)O and its hydrolysis products.

Local Structure and Charge Distribution in the UO2−U4O9 System

Pu L3 X-ray near edge absorption spectra for Pu(0−VII) are reported for more than 60 chalcogenides, chlorides, hydrates, hydroxides, nitrates, carbonates, oxy-hydroxides, and other compounds both as solids and in solution, and substituted in zirconolite, perovskite, and borosilicate glass. This large database extends the known correlations between the energy and shape of these spectra from the usual association of the XANES with valence and site symmetry to higher order chemical effects. Because of the large number of compounds of these different types, a number of novel and unexpected behaviors are observed, such as effects resulting from the medium and disorder that can be as large as those from valence.

Higher Order Speciation Effects on Plutonium L3 X-ray Absorption Near Edge Spectra

Pu L3 X-ray near edge absorption spectra for Pu(0-VII) are reported for more than 60 chalcogenides, chlorides, hydrates, hydroxides, nitrates, carbonates, oxy-hydroxides, and other compounds both as solids and in solution, and substituted in zirconolite, perovskite, and borosilicate glass. This large database extends the known correlations between the energy and shape of these spectra from the usual association of the XANES with valence and site symmetry to higher order chemical effects. Because of the large number of compounds of these different types, a number of novel and unexpected behaviors are observed, such as effects resulting from the medium and disorder that can be as large as those from valence.

Higher Order Speciation Effects on Plutonium L3 X-ray Absorption Near

Information gaps exist in the knowledge base needed for choosing among the alternate processes to be used in the safe conversion of fissile materials to optimal forms for safe interim storage, long-term storage, and ultimate disposition The current baseline storage technology for various wastes uses borosilicate glasses1 The focus of this paper is the synthesis of actinide-containing ceramic materials at low and moderate temperatures (200 °C–1000 °C) using molecular and polymeric actinide borane and carborane complexes

Preparation of actinide boride materials via solid-state metathesis reactions and actinide dicarbollide precursors

The synthesis of actinide tetraborides including uranium tetraboride (UB,), plutonium tetraboride (PUB,) and thorium tetraboride (ThB{sub 4}) by a solid-state metathesis reaction are demonstrated. The present method significantly lowers the temperature required to {approx_equal}850 C. As an example, when UCl{sub 4}, is reacted with an excess of MgB{sub 2}, at 850 C, crystalline UB, is formed. Powder X-ray diffraction and ICP-AES data support the reduction of UCl{sub 3}, as the initial step in the reaction. The UB, product is purified by washing water and drying.

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Anthony J. Lupinetti

Papers

Higher Order Speciation Effects on Plutonium L3 X-ray Absorption Near Edge Spectra

Higher Order Speciation Effects on Plutonium L3 X-ray Absorption Near

Preparation of actinide boride materials via solid-state metathesis reactions and actinide dicarbollide precursors

Low-Temperature Synthesis of Actinide Tetraborides by Solid-State Metathesis Reactions