Structural study and properties of the alkali metal, nitrosyl, and ammonium hepta- and octafluorouranates(VI)

Article abstract of DOI:10.1021/ic50164a043

The thermal decomposition of the heptafluorouranates(VI) of the alkali metals is shown to take place in two steps. The first step gives the octafluorouranates(VI) and UF₆, and the decomposition rate is noticeable at temperatures above 100, 130, 150, and 210°C for the Na, K, Rb, and Cs salts, respectively. The second step for Na₂UF₈ yields pure NaF and UF₆ above 300°C, whereas the decomposition temperatures for the K, Rb, and Cs salts are above 300, 350, and 400°C, respectively. Depending on the decomposition conditions, F₂ and M₂UF₇ (M = K, Rb, Cs) or F₂, UF₆, and M₃UF₈ are formed. The heptafluorouranates(VI) of all the cations studied, except for ammonium, were shown to exhibit dimorphism. The parameters of their cubic form were obtained and are as follows: KUF₇; a = 5.22 A?; RbUF₇; a = 5.385 A?; CsUF₇; a = 5.517 A?; NOUF₇; a = 5.334 A?; NH₄UF₇; a = 5.393 A?; NaUF₇(fccub), a = 8.511 A?, Z = 4. The x-ray pattern of the low-symmetry form of CsUF₇ just below the solid transition temperature (15 ± 1°C) was indexed with a tetragonal cell where a = 5.50 A? and c = 5.37 A?. The x-ray diagrams of the low symmetry form of the other MUF₇ salts were not indexed, whereas those of the octafluorouranates were indexed with orthorhombic cells. The vibrational spectra of the hepta- and octafluorouranates were found to be very dependent on the temperature, and for the same temperature on the cation size. In the solids at high temperature the disordered F positions are likely to be averaged to give pseudo-D_5h and O_h symmetry structures for the UF₇^- and UF₈^2- ions, respectively. At lower temperature, as the motions are frozen out, the observed spectra for the hepta- and octafluorouranates arise from structures of symmetry no higher than C_2ν and D_2d, respectively. The ions UF₇^- and UF₈^2- were characterized in nitrosyl or cesium fluoride HF solutions, which were found to exchange F^- ions with these anions. Based on observation of the chemical exchange between UF₆ and UF₇^- and on a comparative study of the WF₇^- ion, a fluoride ion transfer mechanism is also found for UF₇^- dissolved in acetonitrile. Some trends observed in this study, like the thermal decomposition temperatures or the relative symmetries, are thought to arise from the differences in the cation-anion interaction. This interaction is stronger with smaller cations, which results in more distorted anions, less ionic U-F bonds, and paradoxically less stable complexes.