Neptunyl ions

Neptunyl Ion With Carbonate Ions and Water A Theoretical Study. 

As mentioned in the Introduction, the actinyl ions are not stable under all chemical conditions. Plutonium can coexist in solution in several oxidation states, the stability of which often depends strongly on acidity (26). As a result, great care must be taken to obtain pure solutions of PuOl(27). On the other hand, the neptunyl ion NpO is the most stable form of neptunium in aqueous solution. It is noteworthy that the exchange between the oxygen atoms of PuO and H20 is very slow (ti/2 > 10 h) (25), whereas it is quite fast (h/2 2.2 s) in the case of NpO. ... 

Fig. 72. The neptunyl sulfate framework in structure of (Np 02)2(S04)(H20)2 (a) is based upon 2D sheets shown in (b) (arrangement of neptunyl ions in the sheet is depicted in (c)), and black-and-white graph of the sheet (d).

The structure of (Np 02)2(S04)(H20) (Fig. 73) contains two symmetrically independent Np cations in sevenfold and eightfold coordinations to O atoms. The neptunyl polyhedra share edges to produce chains similar to those observed in the structure of Cs[Np02(Cr04)](H20)2 and which can be considered as elements of the uranophane anion-topology (see above). The chains are linked by cation-cation interactions between the neptunyl ions, and through the sharing of vertices between neptunyl polyhedra and sulfate tetrahedra to form a 3D framework. 

In compounds [Np02(Nic)(H20)] (9) and [Np02(lsonic)(H20)] (10), neptunyl ions act as monodentate ligands with one of 0 atoms entering in the equatorial plane of coordination polyhedron of the adjacent Np atom. The presence of the cation-cation bonds in the structures of 9 and 10 results in the formation of infinite cation-cation chains [Np02(Nic)(H20)] c and [Np02(lsonic)(H20)] c... 

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

Calculated adiabatic excitation energies Ai and experimental values in cm taken from Refs. [155,185] for the free uranyl and neptunyl ions and impurities in CS2UO2CI4 crystal. 

State A free Uranyl ion A impur. Exp. State Neptunyl ion impur. Exp. 

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. 

---