1,1,2,2-Tetrafluorides

Although the number of tetrafluorides reported is as large as the number of di- and trifluorides (see Table III), this group of compounds is the least well characterized structurally of the transition metal fluorides. The synthesis of most of the expected tetrafluorides has been reported, with examples from titanium to manganese in the first, from zirconium to palladium (except for technetium) in the second, and from hafnium to platinum (except for tantalum) in the third series. Many of them have been little studied and, in general, they have not proved amenable to crystallographic structural analysis. 

Compound Space group (symmetry)0 a (At b (At c (A) 0 (degrees) Bond distances (A) Formula unit volume (A3) Reference 

The existence of polymorphs of these tetrafluorides has been questioned more recently (62) and it has been suggested that the reported structures depend on the presence of oxide. Such behavior would parallel that for some of the trifluorides discussed above. 

Bartlett and co-workers (65) have pointed out the relationship of this structure to that of rutile. The MF4 stoichiometry is achieved by an ordered removal of half the metal atoms from the MF2 structure. Each fluorine atom is then either two- (for bridging atoms) or one-coordinate (for terminal atoms), rather than three-coordinate as in the rutile structure. In the rutile arrangement the anions are moved from hexagonal, close-packed positions to have a symmetric arrangement of three metal atom neighbors, with planar coordination. The tetra- 

Although the linking through fluorine bridges is different from that in palladium tetrafluoride, the fluorine atom positions in vanadium tetrafluoride correspond approximately to a hexagonal close-packed array. The linking of octahedra leads to a complex layer structure, rather than the three-dimensional lattice of the palladium tetrafluoride arrangement. 

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Tellurium tetrafluoride, Tep4 m.p. 130 C. Colourless deliquescent solid prepared SeF4 plus TeOj. Hydrolysed by water. 

Vanadium tetrafluoride, VF4. Green solid prepared HF on VCI4. Forms hexa-fluorovanadate(IV) ion (VFa) ". VOF2 formed by hydrolysis. 

Silicon tetrafluoride is formed when hydrogen fluoride reacts with silica or a silicate ... 

Silicon tetrafluoride is a colourless gas, b.p. 203 K, the molecule having, like the tetrahalides of carbon, a tetrahedral covalent structure. It reacts with water to form hydrated silica (silica gel, see p. 186) and hexafluorosilicic acid, the latter product being obtained by a reaction between the hydrogen fluoride produced and excess silicon tetrafluoride ... 

Silicon difluoride is obtained as a very reactive gas when silicon tetrafluoride and silicon are heated together. It polymerises rapidly to give (SiF2) , a solid. 

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 ... 

The structure of sulphur tetrafluoride, and probably also SeF and TeF4, is trigonal bipyramidal with one position occupied by a lone pair of electrons ... 

The action of concentrated sulphuric acid liberates hydrogen fluoride, which attacks glass, forming silicon tetrafluoride the latter is hydrolysed to silicic acid by water, which therefore becomes turbid,... 

Survey and account for the group characteristics and trends in the elements of Group O (He-Rn). Outline the preparation and stereochemistry of xenon tetrafluoride. 

L. silex, silicis, flint) Davy in 1800 thought silica to be a compound and not an element later in 1811, Gay Lussac and Thenard probably prepared impure amorphous silicon by heating potassium with silicon tetrafluoride. 

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. 

Representing a two electron covalent bond by a dash (—) the Lewis structures for hydrogen fluoride fluorine methane and carbon tetrafluoride become... 

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

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