Evidence for a New Composition of Am(IV) Complexes with Tungstophosphate (α₂-P₂W¹⁷O^10- ₆₁and Tungstosilicate (α-SiW₁₁O^8- ₃₉,) Ligands in Nitric Acid Medium

Redox Chemistry of Actinide Ions in Wells−Dawson Heteropolyoxoanion Complexes

“…This shift for the Am reduction potential upon complexation with the WellsϪDawson anion is somewhat larger than the Ϫ0.9 V that is typically seen for similarly complexed An ions. [11,15,21] Were it not for complexation with [α-2-P 2 W 17 10Ϫ is the scientific basis for the separation process known as SESAME [5] and for aspects of the PARC process [6] in nuclear waste treatment operations.…”

“…[11,13Ϫ15] [h] Range of values for pHՅ1. [12,17] [i] Range of values for pH ഠ 7.5 from Baranov et al [16] [j] From Baranov et al [14] A reliable standard potential for the one-electron oxidation of U 4ϩ , unaccompanied by the formation of the linear dioxo moiety [OϭU 5ϩ ϭO] ϩ is not available for comparison with our results. U 5ϩ is unstable, so that, on contact with water, it invariably takes the form of the trans-dioxo cation [UO 2 ] ϩ , which itself is unstable with respect to disproportionation.…”

Americium

“…Chartier et al (1999) Williams et al (2000) used extended X-ray absorption fine structure (EXAFS) to determine that the Am center cation is trivalent when integrated in the Preyssler anion, AmP 5 W 30 O 12À 110 . Sullivan et al (1961) discovered cation-cation interaction between pentavalent and hexavalent actinides.…”

Stabilization of Plutonium(III) in the Preyssler Polyoxometalate

“…The beginnings of trans-uranium polyoxometalate (POM) research can be traced to the Russian Academy of Sciences where, in 1976, Saprykin et al discovered that tetravalent Am and Cm are stabilized by complex formation with the monovacant Wells-Dawson, [P 2 W 17 O 61 ] 10-, and Keggin, [PW 11 O 39 ] 7-, heteropolyoxoanion ligands. 1,2 In their absence, these oxidation states are otherwise reduced to Am(III) and Cm(III) upon contact with water.…”

“…In 1977, this group further demonstrated that tetravalent Np, Pu, and Cf are also stabilized by complexation with [P 2 W 17 O 61 ] 10-. 3,4 Since then, a significant amount of research, [5][6][7] especially that conceived and executed at the Commissariat a `l'Energie Atomique, [8][9][10][11] has demonstrated that tetravalent actinides (An) and lanthanides (Ln) as well (e.g., Pr(IV) and Tb(IV)), which are strong oxidants in the absence of [P 2 W 17 O 61 ] 10-, are stabilized to spontaneous reduction by complex formation with POMs. The stabilization of the small, tetravalent ions with a high charge-to-radius ratio is manifest by the significant negative shift of electrode potentials for the An(IV)/An(III) and Ln(IV)/Ln(III) redox couples in the complex anions with respect to those for the POM-free aqua cations.…”