GROUP V 251
PHOSPHORUS(V) HALIDESThe properties of the phosphorus trihalides given above indicate
the ability of phosphorus to increase its valency above 3. In phos-
phorus pentafluoride, PF 5 (a gas), and the vapour of phosphorus
pentachloride, PC1 5 (solid at ordinary temperatures), phosphorus is
covalently bound to the halogen atoms by five equal bonds to give a
trigonal bipyramid structure. However, the covalency can increase
further to six; the acid, HPF 6 , and its salts, for example NaPF 6 ,
containing the octahedral PF^ ion (hexafluorophosphate) are well
known and stable. Here again then, fluorine excites the maximum
covalency and we can compare the ions A1F|~, SiF^"", PF^.
However, phosphorus pentachloride in the solid state has an
ionic lattice built up of (PC1 4 )+ and (PC1 6 )~ ions and these ions are
believed to exist in certain solvents. Thus under these conditions
the maximum covalency is reached with chlorine. In phosphorus
pentabromide, PBr 5 , the solid has the structure [PBr 4 ] + [Br]~.Phosphorus pentachloride, PC1 5Phosphorus pentachloride is prepared by the action of chlorine
on phosphorus trichloride. To push the equilibrium over to the
right, the temperature must be kept low and excess chlorine must be
present. Hence the liquid phosphorus trichloride is run dropwise
into a flask cooled in ice through which a steady stream of dry
chlorine is passed: the solid pentachloride deposits at the bottom of
the flask.
Phosphorus pentachloride sublimes and then dissociates on
heating, dissociation being complete at 600 K. It is attacked by
water, yielding first phosphorus trichloride oxide, thus :
H 20 + PC1 5 -> 0=PC1 3 + 2HC1 (9.4)
and then tetraoxophosphoric(V) acid:
3H 2 O + POC1 3 -+ H 3 PO 4 + 3HC1 (9.5)
The replacement of the —OH group by a chlorine atom (reaction
9.4) is a very general reaction of phosphorus pentachloride. For
example, if concentrated sulphuric acid is written as (HO) 2 SO 2 then
its reaction with phosphorus pentachloride may be written:
Cl
(HO) 2 SO 2 + 2PC1 5 -» 2O=PC1 3 + 2HC1 + XSO 2
Q\
sulphur dichloride dioxide