Alkaline Metal Fluorooxalatouranilates: Structures and Selected Properties

The structures of crystals Na₃[UO₂(C₂O₄)F₃]⋅4H₂O (I), K₃[UO₂(C₂O₄)F₃] (II), K₃[UO₂(C₂O₄)₂F]⋅3H₂O (III), and Cs[UO₂(C₂O₄)F]⋅H₂O (IV) were studied for the first time using X-ray diffraction. The uranium-containing structural units are complexes [UO₂(C₂O₄)F₃]^3– (for I and II), [UO₂(C₂O₄)₂F]^3– (III), and [UO₂(C₂O₄)F]^– (IV) with crystal chemical formulas А(В⁰¹)M \(_{3}^{1}\) , А(В⁰¹)₂M¹, and А(Q⁰²)M¹ (A = \({\text{UO}}_{{\text{2}}}^{{{\text{2 + }}}}\) ; B⁰¹ or Q⁰² = \({{{\text{C}}}_{{\text{2}}}}{\text{O}}_{{\text{4}}}^{{2 - }}\) ; M¹ = F^–), respectively. In all compounds, the U(VI) atoms implement a pentagonal-bipyramidal coordination; in I–II the uranyl complexes have a mononuclear structure, and in crystals IV they have a chain structure similar to that known for [UO₂(C₂O₄)(H₂O)]⋅2H₂O. The results obtained suggest that the sharp increase in the solubility of uranyl oxalate trihydrate in aqueous solutions with the addition of fluorides is due to the effect of structural depolymerization of coordination polymers in the presence of fluoride ions. A semi-empirical calculation was performed and the calculated and experimental vibration frequencies in the IR spectra of II and IV were compared.