306 GROUP VI
Sulphur hexafluoride, SF 6 , is chemically unreactive, resembling
nitrogen, and is unaffected by heat, water, fused alkalis, and many
heated metals. This stability is attributed to the high S — F bond
strength and to the inability of attacking reagents, such as water, to
coordinate to the covalently saturated sulphur (see SF 4 below). It
finds a use as a high- voltage gaseous insulator.
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:c=o
The structure of sulphur tetrafluoride, and probably also SeF 4 and
TeF 4 , is trigonal bipyramidal with one position occupied by a lone
pair of electrons :
FChlorides
Sulphur and selenium form the chlorides disulphur dichloride S 2 C1 2
and diselenium dichloride Se 2 Cl 2. They are made by the direct
combination of the elements. Both are covalent, yellow liquids
which are readily hydrolysed by water:
S 2 C1 2 + 3H 2 O -> 2HC1 + H 2 S + SO^ + 2H +(Further reaction between hydrogen sulphide and the sulphite ion
yields sulphur together with thionic acids):
2Se 2 Cl 2 + 3H 2 O -> H 2 SeO 3 + 3Se 4- 4HC1