Neptunyl ions complexes

Neptunium(V) exists as a neptunyl ion and either hydrates or complexes (3., k) as shown below ... 

The relativistic calculations on the electronic structure of actinide compounds were reviewed by Pyykko (1987). He also reviewed relativistic quantum chemistry in 1988, whereas the relativistic calculations were limited to small molecules containing one heavy atom only (Pyykko 1988). Calculations on the uranyl and neptunyl ions were introduced in the review article. The general information on the computational chemistry of heavy elements and relativistic calculation techniques appear in the book written by Balasubramanian (1997). There are several first-principle approaches to the electronic structure of actinide compounds. The relativistic effective core potential (ECP) and relativistic density functional methods are widely used for complex systems containing actinide elements. Pepper and Bursten (1991) reviewed relativistic quantum chemistry, while Schreckenbach et al. (1999) reviewed density functional calculations on actinide compounds in which theoretical background and application to actinide compounds were described in detail. The Encyclopedia of computational chemistry also contains examples including lanthanide and actinide elements (Schleyer et al. 1998). The various methods for the computational approach to the chemistry of transuranium elements are briefly described and summarized below. 

Hydroxides. The hydrolysis of Np has been studied more than that of any other pentavalent actinide because it is the most stable oxidation state for Np and it is an actinide ion of significant concern for environmental migration. Pentavalent uranium disproportionates in aqueous solution at pH values where hydrolysis would occur. Hydrolysis products for Pa, Pu, and Am are very similar to, but much less stable than those of Np, so only Np hydroxides will be described in detail. Neptunyl hydrolyzes at about pH 9, to form the stepwise products, Np02(0H) and Np02(0H)2 ", which have been identified by optical absorbance and Raman spectroscopy. " In addition to the hydroxide these complexes likely have two or three inner-sphere waters in the equatorial plane and pentagonal bipyramidal coordination geometry. 

The crystal structure of complex [Np02(Pic)(H20)2] (6) consists of infinite chains situated along [100] direction (Fig. 7). Equatorial plane of each Np atom is formed by one nitrogen atom, two oxygen atoms of two different picolinate-ions and two water molecules. Neptunyl group Np02 is near linear and S5mimetrical with Np-0 distances equal to 1.827(9) and 1.837(9) A. Picolinate-... 

In the case of pentavalent cations, the conditional interaction constant (Table I) obtained for Np02" (log P = 4.6) shows a relatively low affinity of the neptunyl cation with the humic acids as it could be predicted from the charge of the ion. The interaction constant obtained for the hexavalent cation (U) with humic acid (Table I) is independent of pH (4-5) in the non-hydrolysis pH-range but some variation with uranium concentration is observed as for trivalent cations. Moreover, the complexation of uranium to humic substances is of the same order of magnitude than the complexation of trivalent actinides which corroborates chemical analogy between both cations. 

Among the alkylammonium compounds, the tertiary amines are by far the most important actinide extractants, as has already been mentioned (Section 21.5.1). Extraction depends upon the nature and concentration of the anion in much the same manner as for neutral extractants (Section 21.5.2). A good example is the extraction of neptunyl(vi) by methyldioctylamine in chloroform from solutions containing increasing concentrations of sulfuric, hydrochloric, or nitric adds (Fig. 21.9) [178]. The value of D increases rapidly at low concentrations, and most rapidly for sulfuric add, as the complexes formed by the sulfate ion are much stronger than those formed by chloride or nitrate. Consequently, extractable... 

The authors have concluded that hexaphyrin in its oxidized, aromatic form is capable of complexing actinide ions, specifically uranyl (U02 ) and nep-tunyl (Np02 ) neptunyl represents the first crystalographically characterized neptunium complex. The complex is very stable and the authors believe that the system could see application in remediation of radioactive waste. ... 

Electromigration of carrier-free radionuclides. 5. Ion mobilities and hydrolysis of Np(V) in aqueous perchlorate solutions. Radiochim. Acta, 42, 43-46. Runde, W. and Kim, J.L (1994) Chemical Behaviour of Trivalent and Pentavalent Americium in Saline NaCl-Solutions. Studies of Transferability of Laboratory Data to Natural Conditions. Report RCM-01094, Technische Universitat Miinchen, 236 pp. Runde, W., Neu, M.P., and Clark, D.L. (1996) Neptunium(V) hydrolysis and carbonate complexation experimental and predicted neptunyl solubility in concentrated NaCl using Pitzer approach. Geochim. Cosmochim. Acta, 60, 2065-2073. 

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