Actinide cation coordination

Neutral extracting agents possessing oxygen-donor atoms (hard bases) in their structure easily coordinate trivalent lanthanide and actinide cations, but do not discriminate between the two families of elements, because the ion-dipole (or ion-induced dipole type) interactions mostly rely on the charge densities of the electron donor and acceptor atoms. As a result, the similar cation radii of some An(III) and Ln(III) and the constriction of the cation radius along the two series of /elements make An(III)/Ln(III) separation essentially impossible from nitric acid media. They can be separated, however, if soft-donor anions, such as thiocyanates, SCN-, are introduced in the feed (34, 35, 39, 77). 

There are various types of organic proton exchangers (34, 35, 38). Diesters of phosphoric acid, (RO)2P = 0(0H), phosphonic acids, R(RO)P = 0(0H), and phos-phinic acids, R2P = 0(0H), where R represents linear or branched alkyl or phenyl substituents, are the most common cation exchangers developed in liquid-liquid extraction for the extraction of trivalent 4/and 5/elements. They were initially developed for the American TALSPEAK and the Japanese DIDPA processes and have recently been introduced in the French DIAMEX-SANEX process. As for previously described NOPCs, these organophosphorus acids present oxygen-donor atoms (hard bases) in their structures and therefore will easily coordinate trivalent lanthanide and actinide cations, but they will not allow complete discrimination of the two families of elements. However, contrary to previously described neutral organophosphorus... 

Jensen, M.P., Bond, A.H., Rickert, P.G., Nash, K.L. 2002. Solution phase coordination chemistry of trivalent lanthanide and actinide cations with bis(2,4,4-trimeth-ylpentyl)dithiophosphinic acid. Journal of Nuclear Science and Technology (S3) 255-258. 

In most instances, the magnetic structure of a compound can be understood to be based on interacting localized spin centers, such as classical 3d/4d/5d transition metal ions and 4f lanthanide or 5f actinide cations with unpaired electrons. Note that while the assumption of localized moments is valid for many compounds comprising such spin centers, even partial electron delocalization in mixed-valence coordination compounds renders many localized spin models inapplicable. 

Tetravalent. The best-studied tetravalent actinide carbon-ato complex is An(C03)5 (An = Th, U, Pu). This anion has been isolated using a variety of cations, including Na+, K+, T1+, [Co(NH3)6] + and C(NH2)3+/NH4+. In solution, the pentacarbonato complex is the end member of the series An(C03) " " (n = 1-5) however, in the mineral tuhokite, Na6BaTh(C03)6-6H20, thorium exists as a hexacarbonato complex. The analysis of the thermodynamic data for these actinide carbonate systems has led to differences of opinion on the actual speciation. The data appear to support both the stepwise addition of C03 and subsequent loss of H2O molecules within the An + cation coordination sphere as well as the formation of mixed hydroxo carbonato complexes, for example Pu(C03)3(0H) . 

Higher heteroatom coordination numbers are seen for the derivatives of lanthanide and actinide cations, which adopt the structures of [ Ce08 Wio028] (square antiprism, see Square Antiprism) and [ UOnlMonOso] " (icosahedron, see Icosahedron), illustrated in Figures 12 and 13. 

Amines, hydrazines, and hydroxylamines. Amine complexes are known for tetravalent complexes of the earliest actinides (Th, U), particularly for the halides, nitrates, and oxalates. The complexes are generated either in neat amine, or by addition of amine to the parent compound in a nonaqueous solvent. Some of the known simple amine compounds are presented in Table 6. The molecular structure of ThCl4(NMe3)3 has been determined. The coordination environment about the metal is a chloride capped octahedron. A very limited number of adducts exist in which a tetravalent actinide is coordinated by a hydrazine or hydroxylamine ligand the parent compound is generally a halide or sulfate complex. Cationic metal hydrates coordinated with primary, secondary, or tertiary amines have also been isolated with acetylacetonate, nitrate, or oxalate as counterions. 

Actinide cations appecir to behave in a largely ionic manner radius and charge play a predominant role in the determination of structure and coordination t5 es 3). 

While the idea that ligand 7 could prove useful for the coordination of other, non-uranyl actinide cations, has yet to be tested by experiment, it is important to note that this ligand has so far proved less than satisfactory for the coordination of other large, non-actinide cations. Indeed, in spite of extensive efforts devoted to the problem, no stable, non-labile complexes of the trivalent lanthanides (ionic radii 0.86 - 1.36 A ) have as yet been characterized with this system. Nor have 1 1 complexes with other large cations, e.g., Cd + (ionic radius 1.0 A ) or Pb + (ionic radius 1.2 A ), been documented.This has proven to be the case even though mass spectrometric evidence consistent with metal coordination has been obtained in certain instances. 

Solvation numbers of actinide cations in non-aqueous media are almost an unexamined field. FT-IR investigations of the homologous solvates [Ln(N03)3(DMS0) ] in anhydrous acetonitrile (Biinzli etal. 1990) indicated a change in coordination number in the middle of the series [ca. Eu(III)] from nine to eight with increasing atomic number. In the presence of large excess of DMSO([DMSO]j/[Ln]( = 6), the coordination numbers can be increased by one unit. NMR spectroscopy and crystal stoichiometries have indicated a solvation number of two for the uranyl cation in pure TBP (TBP is tributylphosphate). The overall coordination number in this case should include two for TBPs coordination and four for the bidentate nitrate coordination (Siddall and Stewart 1967). 

Carbon monoxide [630-08-0] (qv), CO, the most important 7T-acceptor ligand, forms a host of neutral, anionic, and cationic transition-metal complexes. There is at least one known type of carbonyl derivative for every transition metal, as well as evidence supporting the existence of the carbonyls of some lanthanides (qv) and actinides (1) (see AcTINIDES AND THANSACTINIDES COORDINATION COMPOUNDS). 

The uncertainty of the proper coordination number of any particular plutonium species in solution leads to a corresponding uncertainty in the correct cationic radius. Shannon has evaluated much of the available data and obtained sets of "effective ionic radii" for metal ions in different oxidation states and coordination numbers (6). Unfortunately, the data for plutonium is quite sparse. By using Shannon s radii for other actinides (e.g., Th(iv), U(Vl)) and for Ln(III) ions, the values listed in Table I have been obtained for plutonium. These radii are estimated to have an uncertainty of 0.02 X ... 

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