EDTA and mixed-ligand complexes of tetravalent and trivalent plutonium.

EDTA forms stable complexes with plutonium that are integral to nuclear material processing, radionuclide decontamination, and the potentially enhanced transport of environmental contamination. To characterize the aqueous Pu(4+/3+)EDTA species formed under the wide range of conditions of these processes, potentiometry, spectrophotometry, and cyclic voltammetry were used to measure solution equilibria. The results reveal new EDTA and mixed-ligand complexes and provide more accurate stability constants for previously identified species. In acidic solution (pH < 4) and at 1:1 ligand to metal ratio, PuY (where Y4- is the tetra-anion of EDTA) is the predominant species, with an overall formation constant of log beta110 = 26.44. At higher pH, the hydrolysis species, PuY(OH)- and PuY(OH)(2)2-, form with the corresponding overall stability constants log beta(11 - 1) = 21.95 and log beta(11 - 2) = 15.29. The redox potential of the complex PuY at pH = 2.3 was determined to be E(1/2) = 342 mV. The correlation between redox potential, pH, and the protonation state of PuY- was derived to estimate the redox potential of the Pu(4+/3+)Y complex as a function of pH. Under conditions of neutral pH and excess EDTA relative to Pu4+, PuY(2)4- forms with an overall formation constant of log beta120 = 35.39. In the presence of ancillary ligands, mixed-ligand complexes form, as exemplified by the citrate and carbonate complexes PuY(citrate)3- (log beta1101 = 33.45) and PuY(carbonate)2- (log beta1101 = 35.51). Cyclic voltammetry shows irreversible electrochemical behavior for these coordinatively saturated Pu4+ complexes: The reduction wave is shifted approximately -400 mV from the reduction wave of the complex PuY, while the oxidation wave is invariant.

Chloroheptakis(dimethyl sulfoxide)uranium(IV) trichloride.

In the title complex, [UCl(C(2)H(6)OS)(7)]Cl(3), the uranium metal center is coordinated in a distorted bicapped trigonal prism geometry by seven O atoms from dimethyl sulfoxide ligands and by a terminal chloride ligand. Charge balance is maintained by three outer-sphere chloride ions per uranium(IV) metal center. Principle bond lengths include U-O 2.391 (2)-2.315 (2) A, U-Cl 2.7207 (9) A, and average S-O 1.540 (5) A.

Electrochemical Properties of Bis(dicarbollide)uranium Dibromide.

The cyclic voltammogram of [(eta(5)-C(2)B(9)H(11))(2)UBr(2)].2[Li(THF(4))] shows the electrochemical generation of a uranium(III) species. Subsequent Na/Hg reduction of [(eta(5)-C(2)B(9)H(11))(2)UBr(2)].2[Li(THF(4))] leads to the new isolable uranium(III) species [(eta(5)-C(2)B(9)H(11))(2)UBr(THF)].2[Li(THF)(x)()] (x = 2-4). The green uranium(III) complex was characterized by NMR and elemental analysis and its structure determined by single-crystal X-ray crystallography. The coordination geometry around the dinegative uranium(III) moiety is pseudotetrahedral with two dicarbollide ligands, a bromide ligand, and a THF ligand. The electrochemical properties of the dicarbollide complex are discussed and compared to similar cyclopentadienyluranium complexes.