Comparative investigation of the limiting solution species [U(CO3)5]6- and the crystal structure of Na6[U(CO3)5]12H2O
Comparative investigation of the limiting solution species [U(CO3)5]6- and the crystal structure of Na6[U(CO3)5]12H2O
Hennig, C.; Ikeda-Ohno, A.; Emmerling, F.; Kraus, W.; Bernhard, G.
The coordination of the limiting U(IV) carbonate species in aqueous solution was investigated by comparing its structure parameters with those of the complex preserved in a crystal structure. The solution species was obtained in a aqueous solution of 0.05 M U(IV) and 1 M NaHCO3. Single crystals of Na6[U(CO3)5]12H2O were obtained directly from this mother solution. The U(IV) carbonate complex in the crystal structure was identified as [U(CO3)5]6 anionic complex. This monomeric complex forms a network with charge compensating Na+ cations and H2O ligands. The interatomic distances around the U(IV) coordination polyhedron show average distances of U-O = 2.461(8) Å, U-C = 2.912(4) Å and U-Odist = 4.164(6) Å. U L3edge EXAFS spectra were collected from the solid Na6[U(CO3)5]12H2O and the corresponding solution. In both samples, the first shell of the Fourier transforms (FTs) revealed ten oxygen atoms at an average distance of 2.450.02 Å, the second shell originates from five carbon atoms with a U-C distance of 2.910.02 Å, and the third shell was fit with single and multiple scattering paths of the distal oxygen at 4.170.02 Å. These data indicate the identity of the [U(CO3)5]6 complex in solid and solution state. The high negative charge of the [U(CO3)5]6 anion is compensated by Na+ cations. In solid state the Na+ cations form a bridging network between the [U(CO3)5]6 units, while in liquid state they seem to be located closer at the anionic complex. The average metal-oxygen distances of the coordination polyhedron show a linear correlation to the radius contraction of the neighbour actinide(IV) ions and indicate the equivalence of the [An(CO3)5]6 coordination within the series of thorium, uranium, neptunium and plutonium.
Keywords: U(IV) carbonate species; EXAFS; XRD
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Dalton Transactions 39(2010), 3744-3750
DOI: 10.1039/b922624a
Cited 40 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ-12900