The structure and bonding of Y, Eu, U, Am and Cm complexes as studied by quantum chemical methods and X-ray crystallography

Abstract

Two isomers of Y/Eu(TTA)₃(OH₂) complexes and their structures were identified by single crystal X-ray diffraction, and their geometry and bond distances were also determined using quantum chemical (QM) methods. The data from the two methods agree very well, suggesting that QM is appropriate for calculating structures for which no experimental data are available. This method was therefore used to determine the structures of U/Am/Cm(TTA)₃(OH₂)₂ both in gas phase and in CPCM models of CCl₄, CHCl₃, and H₂O. In these calculations the metal sites were described using the f-in-core approximation, comparing small- and large-core pseudopotentials (SPP and LPP) with their corresponding segmented basis sets. The difference between the Y/Eu–O bond distances between the LPP and SPP is less than 0.02 Å. However, in the actinide complexes the LPP results in larger An–O distances and the difference between the LPP and SPP results decreases from about 0.20 Å in the U-complex to 0.05 Å in the Am- and 0.04 Å in the Cm-complex. The chemical bonding studied by population analysis indicates that there is a significant back bonding in U(TTA)₃(OH₂)₂ from filled orbitals centered on uranium into empty π* orbitals on coordinated oxygen; there is some evidence of back-bonding also in the americium complex, but a significantly smaller effect in the europium and curium species. The relative energy of the two isomers indicates they may be present in solution in comparable amounts, suggesting the possibility of exchange between them. The X-ray structures suggest two exchange pathways, a topological “twist” mechanism and a site exchange involving the opening of a TTA chelate ring.