Neptunyl complexes

AH2(Pu02Cl2) = 18.3 + 4.2 kJ.mol l. These values lead to aSi(Pu02 C1+) = 32 + 15 J.K-T mol-1 and aS2(Pu02C12) = 53+15 J.K-i.mol-1, results whTch are not unreasonable. Comparison of these results (3, aH. ..) with the numerous data existing on the uranyl and neptunyl complexes does not reveal smooth trends so that discussion of this topic cannot be carried out here. 

On the other hand, the intensities of 5/ - 5/ transitions in neptunyl complexes are strongly affected by both the number of ligands in the equatorial... 

The neptunyl complexes with TPTZ were studied in the work [98]. Two solid compounds, [Np02Cl(TPTZ)(H20)] H2O and [Np02Br(TPTZ)(H20)] 2H2O, were synthesized, and the crystal structure of the first complex was determined using X-ray diffractometry. 

Fig. 25. Molecular structure of [Np02(MIDA)2] anion in neptunyl complex with N-methyliminodiacetic acid C(NH2)3 3[Np02(MIDA)2].

We have also studied the coordination of the monocarbonate, bicarbonate, and tricarbonate complexes of neptunyl in water, by using both explicit water molecules and a continuum solvent model.84 The monocarbonate complex was shown to have a pentacoordinated structure, with three water molecules in the first coordination shell, and the bicarbonate complex has a hexacoordinated structure, with two water molecules in the first coordination shell. Overall good agreement with experimental results was obtained. 

In contrast, the solubilities of the neptunyl species were increased by HEDTA and unexpectedly decreased by EDTA. Plutonyl(V) solubility was not affected by HEDTA or EDTA. The component most significant to neptunium solubility was NaNOz. Sodium nitrite apparently decreased neptunium solubility by reducing the more soluble neptunyl(VI) to the less soluble (V) state. The existence of two oxidation states complicated interpretation of the neptunium data. Sodium carbonate, phosphate, aluminate, and sulfate all increased neptunium solubility, probably through complexation. Separation of the data... 

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. 

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