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Issue 15, 2010
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Comparative investigation of the solution species [U(CO3)5]6− and the crystal structure of Na6[U(CO3)5]·12H2O

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Abstract

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 prevails in aqueous solution of 0.05 M U(IV) and 1 M NaHCO3 at pH 8.3. 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 a monomeric [U(CO3)5]6− anionic complex. 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 L3–edge EXAFS spectra were collected from the solid Na6[U(CO3)5]·12H2O and the corresponding solution. The first shell of the Fourier transforms (FTs) revealed, in both samples, a coordination of ten oxygen atoms at an average U–O distance of 2.45 ± 0.02 Å, the second shell originates from five carbon atoms with a U–C distance of 2.91 ± 0.02 Å, and the third shell was fit with single and multiple scattering paths of the distal oxygen at 4.17 ± 0.02 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 the Na+ cations seem to be located close to the anionic complex. The average metal-oxygen distances of the coordination polyhedron show a linear correlation to the radius contraction of the neighbouring actinide(IV) ions and indicate the equivalence of the [An(CO3)5]6− coordination within the series of thorium, uranium, neptunium and plutonium.

Graphical abstract: Comparative investigation of the solution species [U(CO3)5]6− and the crystal structure of Na6[U(CO3)5]·12H2O

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Publication details

The article was received on 28 Oct 2009, accepted on 28 Jan 2010 and first published on 09 Mar 2010


Article type: Paper
DOI: 10.1039/B922624A
Dalton Trans., 2010,39, 3744-3750

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    Comparative investigation of the solution species [U(CO3)5]6− and the crystal structure of Na6[U(CO3)5]·12H2O

    C. Hennig, A. Ikeda-Ohno, F. Emmerling, W. Kraus and G. Bernhard, Dalton Trans., 2010, 39, 3744
    DOI: 10.1039/B922624A

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