Modern inorganic chemistry

GROUP IV 195

DILEAD(I1)LEAD(1V) OXIDE, 'RED LEAD', Pb 3 O 4

Red lead is a brilliant red powder obtained by heating lead mon-

oxide in air to about 800 K. This reaction is reversible, for if heated,

red lead evolves oxygen at temperatures above 850 K.

6PbO 4- O 2 ^ 2Pb 3 O 4

Red lead is insoluble in water. Like lead(II) oxide it can readily be

reduced to lead. The structure of the solid, as the systematic name

suggests, consists of two interpenetrating oxide structures, in which

each PbIV atom is surrounded octahedrally by six oxygen atoms,

and each Pb° by three (pyramidal) oxygen atoms, the oxygen atoms

being shared between these two units of structure. With dilute nitric

acid the lead(II) part dissolves, and the lead(IV) part precipitates as

lead(IV) oxide:

Pb 2 [PbO 4 ] + 4HNO 3 -> 2Pb(NO 3 ) 2 + PbO 2 i + 2H 2 O

Red lead is a useful ingredient of anti-rusting paints, in which it is

mixed with linseed oil. If glycerol is added to this mixture, a cement

suitable for luting (i.e. making airtight or watertight) joints in iron

pipes or vessels is obtained.

CHLORIDES AND OTHER IMPORTANT HALIDES OF

GROUP IV ELEMENTS

All Group IV elements form tetrachlorides, MX 4 , which are pre-

dominantly tetrahedral and covalent. Germanium, tin and lead also

form dichlorides, these becoming increasingly ionic in character as

the atomic weight of the Group IV element increases and the

element becomes more metallic. Carbon and silicon form catenated

halides which have properties similar to their tetrahalides.

Carbon

When carbon forms four covalent bonds with halogen atoms the
second quantum level on the carbon is completely filled with
electrons. Most of the reactions of the Group IV tetrahalides
require initial donation by a Lewis base (p. 91) (e.g. water, ammonia)
which attaches initially to the tetrahalide by donation of its electron
pair. Hence, although the calculated free energy of a reaction may
indicate that the reaction is energetically favourable, the reaction
may still not proceed. Thus we find that the tetrahalides of carbon