Synthèse et caractérisation des oxypentafluorouranates(VI) NH 4 UOF 5 , KUOF 5 , RbUOF 5 et CsUOF 5

International audienceThe oxypentafluorouranates(VI) MUOF 5 , where M = NH 4 , K, Rb, Cs, have been synthetized from reaction of UOF 4 with the ammonium or corresponding alkali metal fluoride in liquid SO 2 . According to X-ray diffraction, Raman and infrared spectroscopy, and from an isomorphism with the corresponding hexafluorouranates(V) MUF 6 , two different environments around the uranium atom are observed. In CsUOF 5 the five fluorine atoms and the oxygen around the uranium result in a pseudo-octahedral surrounding whereas for the other complexes (M = NH 4 , K, Rb) each uranium is surrounded by eight light atoms forming a dodecahedron. In this structure the dodecahedra are linked together by fluorine atoms to form infinite chains. The UOF 5 − ion has been characterized by vibrational spectroscopy in the solid state. The proposed assignment, which was made with the assumption of a C 4v symmetry of the UOF 5 − ion, was confirmed by a force constant calculation. From these data and contrary to the values reported for comparable oxypentafluoroanions, the axial fluorine is found to be less ionic than the equatorial ones