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

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–O_dist = 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.