Direct synthesis of anhydrous alkali-metal tetrafluorodioxouranates(VI), A2[UO2F4], and the first synthesis of alkali-metal diaquatetrafluorodioxouranate(VI) monohydrates, A2[UO2F4(H2O)2]·H2O (A = Na, K or NH4)

Abstract

Anhydrous alkali-metal tetrafluorodioxouranates(VI), A₂[UO₂F₄](A = Na, K or NH₄), have been synthesised directly from the reaction of [UO₂(O₂)]·2H₂O with aqueous HF and alkali-metal fluorides AF (A = Na, K or NH₄). The yellow product obtained by addition of an alkali to an aqueous solution of [UO₂(NO₃)₂]·6H₂O yielded, with aqueous HF and alkali-metal carbonates, the alkali-metal diaquatetrafluorodioxouranate(VI) monohydrates, A₂[UO₂F₄(H₂O)₂]·H₂O (A = Na, K or NH₄). The compounds are crystalline. Infrared and laser Raman spectroscopic investigations provided the basis of their structural assessment. The solution electrical conductance (ca. 242 Ω^–1 cm² mol^–1) attests to their 2 : 1 ionic nature and stability in aqueous solution. Pyrolysis of [NH₄]₂[UO₂F₄(H₂O)₂]·H₂O at 120 °C followed by deuteriation and IR spectroscopy of the product provided evidence for the occurrence of a molecule of lattice water and two aqua ligands.