Modern inorganic chemistry

THE TRANSITION ELEMENTS 371

CHLORIDES

The important halide is the tetrachloride, TiCl 4 , made from the

dioxide as already stated (p. 370). It is a colourless volatile liquid,

b.p. 409 K, readily hydrolysed by water (see below); the Ti—Cl

bonds are covalent, and the molecule is monomeric and tetrahedral

(cf. the halides of Group IV). It dissolves in concentrated hydro-

chloric acid to give the hexachlorotitanate(IV) anion, [TiCl 6 ]^2 ~ ;

salts of this anion can be precipitated from the solution by addition

of an alkali metal chloride, for example KC1 gives K 2 TiCl 6 (compare

again the behaviour of the Group IV halides). The [TiCl 6 ]^2 ~ ion

has an octahedral configuration and is the simplest representative of

a large number of titanium(IV) complexes, of general formula

[TiX 6 ]"~, where X represents a number of possible ligands and

n = 0, 1 or 2. This ability of TiX 4 compounds to increase their

coordination to TiX 6 has an important practical use. If trimethyl-

aluminium, (CH 3 ) 3 A1, is added to a solution of titanium tetrachloride

and an olefin such as ethylene passed into the mixture, the olefin is

readily polymerised. This is the basis of the Ziegler-Natta process for

making polyolefins, for example 'polypropylene', and the mechan

ism is believed to involve the coordination of the olefin to molecules

ofthetypeCH 3 TiCl 3.

Titanium tetrachloride is hydrolysed by water, to give a mixture

of anions, for example [Ti(OH)Cl 5 ]~ and [TiCl 6 ]^2 ~, together with

some hydrated titanium dioxide (TiO 2 ,4H 2 O is one possible hydrate,

being equivalent to [Ti(OH) 4 (H 2 O) 2 ]). This suggests that titanium

dioxide is amphoteric (see below).

TITANIUM DIOXIDE

This occurs naturally as a white solid in various crystalline forms,
in ail of which six oxygen atoms surround each titanium atom.
Titanium dioxide is important as a white pigment, because it is non-
toxic, chemically inert and highly opaque, and can be finely ground:
for paint purposes it is often prepared pure by dissolving the natural
form in sulphuric acid, hydrolysing to the hydrated dioxide and
heating the latter to make the anhydrous form.
Anhydrous titanium dioxide is only soluble with difficulty in hot
concentrated sulphuric acid; dilution allows the crystallisation of
a sulphate of formula TiOSO 4 .H 2 O, but it is doubtful if the ktitanyF
cation TiO^2 + actually exists, either in solution or the solid. Certainly
[Ti(H 2 O)n]4+ does not exist, and solutions of "titanyl' salts may
best be considered to contain ions [Ti(OH) 2 (H 2 O 4 )]^2 +. Titanium