Structure and dynamics in the complex ion (UO2)2(CO3)(OH)3−

Abstract

The structure and ligand exchange dynamics of the ternary complex (UO₂)₂(CO₃)(OH)₃⁻ have been investigated by EXAFS and NMR spectroscopy. Very broad signals can be observed in both the ¹³C and the ¹⁷O NMR spectra. The EXAFS data show the presence of 1.3 ± 0.3 short uranium–oxygen distances at 2.26 Å, consistent with single bonded hydroxide and 3.9 ± 0.6 distances at 2.47 Å for the other ligands in the first co-ordination shell. There is also evidence for a U⋯U interaction at 3.90 Å. Based on the EXAFS and NMR data we suggest the presence of three isomers with different bridge arrangements, the dominant one, C, contains 80% of the uranium and the minor ones A and B, 5 and 15%, respectively. The ligand exchange reactions between these isomers are slow. The NMR data indicate that the main reactions involve intramolecular exchanges between isomers with different positions of the non-bridging ligands in A, B and C. We suggest that these take place through water exchange as discussed earlier for other ternary uranium(VI) complexes.