Hexafluorides

It has been suggested (79) that polymorphism may occur for transition metal pentafluorides as it does for oxide tetrafluorides (91). X-Ray powder photographs of products from the reduction of hexafluorides in anhydrous hydrogen fluoride (79) showed that the samples of rhenium and osmium pentafluorides had different structures from those previously reported, but no unit-cell dimensions could be derived. 

The formation of dimolybdenum nonafluoride, formally MoF4, MoF5, has been reported (78), but no structural data were obtained. 

Chromium is the only metal of the first transition series to form a hexafluoride (3). The compound is so unstable that no solid-state characteristics have been recorded. The metals of the second and third transition series form hexafluorides which, structurally as a group, are the most closely related of all the transition metal fluorides. These compounds are molecular species, with an essentially regular, octahedral arrangement of fluorine atoms around the metal atom. This leads 

The structure of tungsten hexafluoride showing the approximate double-hexagonal close packing of fluorine atoms. Hatched circles show atoms at x = 0, double circles those at x = j, single circles those at x - 5, and crossed circles those at x = corresponding to the sequence ABAC. Metal atoms are shown as small, dashed circles. 

All of these hexafluorides are dimorphic, with a high-temperature, cubic form and an orthorhombic form, stable below the transition temperature (92). The cubic form corresponds to a body-centered arrangement of the spherical units, with very high thermal disorder of the molecules in the lattice, leading to a better approximation to a sphere. Recently, the structures of the cubic forms of molybdenum (93) and tungsten (94) hexafluorides have been studied using neutron powder data, with the profile-refinement method and Kubic Harmonic analysis. In both compounds the fluorine density is nonuniformly distributed in a spherical shell of radius equal to the M—F distance. Thus, rotation is not completely free, and there is some preferential orientation of fluorine atoms along the axial directions. The M—F distances are the same as in the gas phase and in the orthorhombic form. 

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Selenium hexafluoride, SeF m.p. — 39°C, sublimes —47°C. Formed (with Sc2Fio) from Se and F2- Chemically fairly inert. 

Sulphur hexafluoride, SF, m.p. — 5LC. Formed S plus Fj. Very inert material used as an inert dielectric. S2F10 (toxic) is also formed from S plus F2 and there is an extensive chemistry of SFj derivatives (e.g. SF5CI, CIF plus SFJ. 

All three elements form gaseous hexafluorides by the direct combination of the elements. They all have octahedral structures... 

Both selenium hexafluoride and tellurium hexafluoride are more reactive than sulphur hexafluoride. Tellurium hexafluoride is slowly hydrolysed by water to telluric) VI) acid and on heating it decomposes to fluorine and the tetrafluoride. 

The tetrafluorides of the elements can be prepared. They are all less stable than the corresponding hexafluorides and are hydrolysed readily by water. They can all be used as fluorinating agents and sulphur tetrafluoride is extensively used for this purpose, for example the fluorination of organic carbonyl groups ... 

Fluorine and its compounds are used in producing uranium (from the hexafluoride) and more than 100 commercial fluorochemicals, including many well known high-temperature plastics. Hydrofluoric acid etches the glass of light bulbs, etc. Fluorochlorohydrocarbons are extensively used in air conditioning and refrigeration. 

We also developed a number of other useful new fluorinating reagents. They ineluded a convenient in situ form of sulfur tetrafluoride in pyridinium polyhydrogen fluoride, selenium tetrafluoride, and ey-anurie fluoride. We introdueed uranium hexafluoride (UFg), depleted from the U-235 isotope, which is an abundant by-product of enrichment plants, as an effective fluorinating agent. 

IV) oxide sulfate (III) sulfate (III) sulfide Xenon difluoride hexafluoride tetrafluoride trioxide Ytterbium... 

Toluene Sulfuric plus nitric acids, nitrogen dioxide, silver perchlorate, uranium hexafluoride... 

If a molecule has a centre of inversion (or centre of symmetry), i, reflection of each nucleus through the centre of the molecule to an equal distance on the opposite side of the centre produces a configuration indistinguishable from the initial one. Figure 4.4 shows s-trans-buta-1,3-diene (the x refers to trans about a nominally single bond) and sulphur hexafluoride, both of which have inversion centres. 

Figure 4.4 Inversion centre, i, in (a) s-traws-buta-f, 3-diene and (b) sulphur hexafluoride...

Figure 5.1 Principal inertial axes of (a) hydrogen cyanide, (b) methyl iodide, (c) benzene, (d) methane, (e) sulphur hexafluoride, (f) formaldehyde, (g) s-lraws-acrolein and (h) pyrazine...

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