Pentavalent actinides

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

Americium may be separated from other elements, particularly from the lanthanides or other actinide elements, by techniques involving oxidation, ion exchange and solvent extraction. One oxidation method involves precipitation of the metal in its trivalent state as oxalate (controlled precipitation). Alternatively, it may be separated by precipitating out lanthanide elements as fluorosilicates leaving americium in the solution. Americium may also he oxidized from trivalent to pentavalent state by hypochlorite in potassium carbonate solution. The product potassium americium (V) carbonate precipitates out. Curium and rare earth metals remain in the solution. An alternative approach is to oxidize Am3+ to Am022+ in dilute acid using peroxydisulfate. Am02 is soluble in fluoride solution, while trivalent curium and lanthanides are insoluble. 

Analysis of the Absorption Spectra of Complex Pentavalent Actinide Halides. LiUF6, o -NaUF6 and CsUFg. H.G. Hecht, J.G. Malm, J. Foropoulus, and W.T. Camall, J. Chem. Phys. 84, 3653-3662 (1986). 

The majority of the chemistry that has been investigated for the actinide elements has been in aqueous solutions. For the light actinides in acidic solutions, four types of cations persist trivalent, tetravalent, pentavalent, and hexavalent. The later two ions are always found to have trans oxo ligands, making up a linear dioxo unit. Actinide ions of this type are typically referred to yls and have the names, uranyl (1, U02+/ +), neptunyl (2, Np02+ +), plutonyl (3, Pu02+ +), (4, Am02+/ ) and so on. 

For those actinides stable in the pentavalent state, ions of the form An02(H20)x+ (An = Np, Pu x = 4, 5, 6) have been postulated. Pa is not included in this list since it readily hydrolyzes in aqueous solutions to form Pa205-xH20. All of the structural data for these complexes come from XAFS. The bond distances for both An=0 (1.83 A for both Np and Pu) and An-OH2 (2.51 and 2.5 A, respectively) expand in the... 

Pentavalent. The binding of carbonate to pentavalent actinide ions has been studied for most of the light actinides (An = U, Np, Pu, Am). The primary species that... 

Pentavalent. A limited number of pentavalent actinide complexes have been isolated with N-donor ligands. The primary reason for this is the inability of Th to attain the pentavalent state and the propensity of U complexes to disproportionate into and complexes. Some of the typical complexes that have been reported are adducts of U(0R)5 and UX5 with the hgands described for the tri- and tetravalent actinide complexes. 

Crystallographic Properties of Some Pentavalent Actinide Hexafluoro Complexes ... 

Arsines. The ligand o-phenylenebis(dimethylarsine) (diars) has been used to complex actinide halides. The complexes PaCl4(diars) and UCl4(diars) have been reported. Both are produced by the reduction of pentavalent precursors in solution upon addition of the arsine. ... 

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 pentavalent oxidation state is accessible for the early actinides uranium, protactinium, neptunium, and plutonium. Pentavalent species with neutral Group 16 bases can include either adducts of AnXg or complexes incorporating oxo-containing cations, AnO " or An02". ... 

Ethers. The only reported ether compounds of pentavalent actinides are the species UCls ether, where ether = THE or R2O (R = Me, Et, Pr , Bu", and i-CsHn). Dioxane is suggested to form either both 1 1 and 1 3 (U L) adducts. ... 

Hydroxamate. Similar to trivalent hydroxamates, pentavalent actinide hydroxamate complexes are generally unstable relative to tetravalent and/or hexavalent complexes. Pu or Pu hydroxamate complexes can be prepared however, at near-neutral and basic pH they rapidly reduce to Pu complexes. ... 

Pentavalent actinides can also exhibit a dioxo ion, (An02), but no structures of inorganic phosphate or arsenate compounds with this moiety have yet been reported, although this is an area of ongoing research [100]. 

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