GROUPV 249
HALOGEN COMPOUNDS OF GROUP V ELEMENTSNitrogen trifluoride and trichloride can both be prepared as pure
substances by the action of excess halogen on ammonia, a copper
catalyst being necessary for the formation of nitrogen trifluoride.
Nitrogen trifluoride is an exothermic compound (A/ff = — 124.7
kJ moF^1 ). It is an unreactive gas with a high thermal stability and a
very low dipole moment (cf. NH 3 , p. 216).
In contrast the endothermic trichloride, A/ff = + 230.1 kJ
moP^1 ), is extremely reactive with a tendency to explode, being
particularly unstable above its boiling point, 344 K, in light, or in
the presence of organic compounds. Unlike the trifluoride it is
readily hydrolysed by water to ammonia and chloric(I) acid:
NC1 3 4- 3H 2 O -> NH 3 + 3HOC1
The pure tribromide and triodide are unknown but their ammoniates
have been prepared by the action of the appropriate halogen on
ammonia. The tribromide, NBr 3 6NH 3 , is a purple solid which
decomposes explosively above 200K. The iodide, NI 3 .wNH 3 , is a
black explosive crystalline solid, readily hydrolysed by water.Phosphorus, arsenic, antimony and bismuth
With the exception of phosphorus trifluoride, these elements form
their trihalides by direct combination of the elements, using an
excess of the Group V element. As a series they show increasing
ionic character from phosphorus to bismuth, this being indicated
by their increasingly higher melting and boiling points and their
increasing ability to form cations in aqueous solution. In addition
to the trihalides a number of pentahalides have also been prepared.
All the pentafluorides are known, together with the pentachlorides
of phosphorus, and antimony. Phosphorus also forms a penta-
bromide. Some of the important halides are discussed in more detail
below.
THE PHOSPHORUS(III) HALIDES
Phosphorus trifluoridePhosphorus trifluoride is a colourless gas; the molecule has a
shape similar to that of phosphine. Although it would not be
expected to be an electron donor at all (since the electronegative