Modern inorganic chemistry

250 GROUP V

fluorine atoms will attract the lone pair electrons), it forms a com-

pound with nickel, Ni(PF 3 ) 4 , very like nickel tetracarbonyl, Ni(CO) 4 ,

This is explained by the fact that phosphorus can expand its valency

shell of electrons and so receive electrons from the nickel by a kind

of 'back-donation', i.e. each nickel-phosphorus bond is Ni=PF 3 ,

not just Ni<-PF 3.

Phosphorus trichloride

Phosphorus and chlorine combine directly to form either the tri-

chloride or the pentachloride depending on the relative amounts of

phosphorus and chlorine used.

The trichloride is obtained as a liquid, boiling point 349 K, when

a jet of chlorine burns in phosphorus vapour. Care must be taken

to exclude both air and moisture from the apparatus since phos-

phorus trichloride reacts with oxygen and is vigorously hydrolysed

by water, fuming strongly in moist air. The hydrolysis reaction is :

PC1 3 + 3H 2 O -» H 3 PO 3 + 3HC1

phosphonic

acid

Similar reactions occur with organic compounds which contain

hydroxyl groups, thus

3CH 3 C + PC1 3 -* 3CH 3 C + H 3 PO 3

OH Cl

Hydrogen chloride is also evolved.

The reaction with oxygen converts phosphorus trichloride to

phosphorus trichloride oxide (oxychloride), POC1 3 ; the trichloride

is able to remove oxygen from some molecules, for example sulphur

trioxide

PC1 3 + SO 3 -* O = P-C1 + SO 2

Phosphorus trichloride reacts with chlorine in excess to give phos-
phorus pentachloride, an equilibrium being set up :

PC1 3 + C1 2 ^ PCL