STRUCTURE, ISOMERISM, AND LIGAND DYNAMICS IN DIOXOURANIUM(VI) COMPLEXES

The ligand exchange reactions in complexes of the type UO2LFn(H2O)(3-n ), where n = 1-3 and L is one of the bidentate ligands picolinate (pic ), oxalate (ox), carbonate, and acetate (ac), UO2L2F (L = picolinate o r oxalate), have been investigated by using an array of F-19-, C-13-, O-17-, and H-1-NMR techniques. The rates of exchange were sufficiently low to allow the identification of isomers,which are those expected f or a pentagonal bipyramid coordination geometry, with the ''yl'' oxyge ns in the apical positions. All ligand exchange reactions were indepen dent of the concentration of free ligand. Two different fluoride excha nge pathways were identified, intermolecular exchange between coordina ted and free fluoride and intramolecular exchange between different fl uoride sites within a given complex. The intramolecular exchange react ion in UO2(pic)F-3(3-) was shown to be a result of opening/closing of the chelate ring, equivalent to a site exchange between the two ''edge '' fluorides. Dissociation of acetate is the major factor determining the intramolecular fluoride exchange reactions in UO2(ac)F-3(2-). In t his same way the broad F-19-NMR peaks or UO2LF2(H2O)(2-) and UO2LF(H2O )(2)(-) are a result of a comparatively slow exchange between coordina ted and bulk water. At pH > 6 UO2(ox)F(H2O)(2)(-) forms a hydroxide-br idged dimer (UO2)(2)(ox)(2)F-2(OH)(2)(4-), which exists in two isomeri c forms.