Neptunium compounds
Neptunium has five ionic oxidation states ranging from +3 to +7 when forming chemical compounds, which can be simultaneously observed in solutions.
Unlike its neighboring homologues UO+2 and PuO+2, NpO+2 does not spontaneously disproportionate except at very low pH and high concentration:[4] It hydrolyzes in basic solutions to form NpO2OH and NpO2(OH)−2.
[7] Np(VI) or NpO2+2, the neptunyl ion, shows a light pink or reddish color in an acidic solution and yellow-green otherwise.
Though its chemical formula in basic solution is frequently cited as NpO3−5, this is a simplification and the real structure is probably closer to a hydroxo species like [NpO4(OH)2]3−.
[3][7] Neptunium(III) hydroxide is quite stable in acidic solutions and in environments that lack oxygen, but it will rapidly oxidize to the IV state in the presence of air.
[7][10][11][12] Three anhydrous neptunium oxides have been reported, NpO2, Np2O5, and Np5O8, though some studies[13] have stated that only the first two of these exist, suggesting that claims of Np5O8 are actually the result of mistaken analysis of Np2O5.
This behavior is illustrated by the fact that NpO2 can be produced by simply burning neptunium salts of oxyacids in air.
Additional compounds have been produced by reacting NpO3 and water with solid alkali and alkaline peroxides at temperatures of 400 - 600 °C for 15–30 hours.
[19][20][21][22][23][24][25][26] A large number of additional alkali and alkaline neptunium oxide compounds such as Cs4Np5O17 and Cs2Np3O10 have been characterized with various production methods.
Neptunium has also been observed to form ternary oxides with many additional elements in groups 3 through 7, although these compounds are much less well studied.
[19][27][28] Although neptunium halide compounds have not been nearly as well studied as its oxides, a fairly large number have been successfully characterized.
Of these, neptunium fluorides have been the most extensively researched, largely because of their potential use in separating the element from nuclear waste products.
The first two are fairly stable and were first prepared in 1947 through the following reactions: Later, NpF4 was obtained directly by heating NpO2 to various temperatures in mixtures of either hydrogen fluoride or pure fluorine gas.
This volatility has attracted a large amount of interest to the compound in an attempt to devise a simple method for extracting neptunium from spent nuclear power station fuel rods.
[41][42][43] Neptunium chalcogen and pnictogen compounds have been well studied primarily as part of research into their electronic and magnetic properties and their interactions in the natural environment.
Pnictide and carbide compounds have also attracted interest because of their presence in the fuel of several advanced nuclear reactor designs, although the latter group has not had nearly as much research as the former.
The first has a face centered cubic structure and is prepared by converting neptunium metal to a powder and then reacting it with phosphine gas at 350 °C.
It may be obtained from the reaction of neptunium hydride with graphite at 1400 °C or by heating the constituent elements together in an electric arc furnace using a tungsten electrode.
[54][55][61][62] Neptunium reacts with hydrogen in a similar manner to its neighbor plutonium, forming the hydrides NpH2+x (face-centered cubic) and NpH3 (hexagonal).
The hydrides require extreme care in handling as they are explosive and also decompose in a vacuum at 300 °C to form finely divided neptunium metal, which is pyrophoric.
It is isomorphous to uranocene and plutonocene, and they behave chemically identically: all three compounds are insensitive to water or dilute bases but are sensitive to air, reacting quickly to form oxides, and are only slightly soluble in benzene and toluene.
[66] Other known neptunium cyclooctatetraenyl derivatives include Np(RC8H7)2 (R = ethanol, butanol) and KNp(C8H8)·2THF, which is isostructural to the corresponding plutonium compound.
[67] The coordination chemistry of neptunium(V) has been extensively researched due to the presence of cation–cation interactions in the solid state, which had been already known for actinyl ions.
[67] Extensive study has been performed on compounds of the form M4AnO2(CO3)3, where M represents a monovalent cation and An is either uranium, neptunium, or plutonium.
The first reported such compound was initially characterized as Co(NH3)6NpO5·nH2O in 1968, but was suggested in 1973 to actually have the formula [Co(NH3)6][NpO4(OH)2]·2H2O based on the fact that Np(VII) occurs as [NpO4(OH)2]3− in aqueous solution.
[67] This compound forms dark green prismatic crystals with maximum edge length 0.15–0.4 mm.
[68] A great many complexes for the other neptunium oxidation states are known: the inorganic ligands involved are the halides, iodate, azide, nitride, nitrate, thiocyanate, sulfate, carbonate, chromate, and phosphate.
Many organic ligands are known to be able to be used in neptunium coordination complexes: they include acetate, propionate, glycolate, lactate, oxalate, malonate, phthalate, mellitate, and citrate.
[68] Analogously to its neighbours, uranium and plutonium, the order of the neptunium ions in terms of complex formation ability is Np4+ > NpO2+2 ≥ Np3+ > NpO+2.
[68] NpO+2 can also form the complex ions [NpO+2M3+] (M = Al, Ga, Sc, In, Fe, Cr, Rh) in perchloric acid solution: the strength of interaction between the two cations follows the order Fe > In > Sc > Ga > Al.[68] The neptunyl and uranyl ions can also form a complex together.
