372 THE TRANSITION ELEMENTS
dioxide is not soluble in aqueous alkali, but with fused alkali gives
a titanate, for example
2KOH + TiO 2 -> K 2 TiO 3 4- H 2 O
Hence titanium dioxide is clearly amphoteric.Oxidation state + 3In this oxidation state the outer electronic configuration is 3d^1 , so
the compounds are necessarily paramagnetic (p. 229) and are
coloured.TitaniurnHHI) chloride, TiCl 3 , is made by reduction of the tetra-
chloride with, for example, hydrogen. In the anhydrous form it has
a covalent polymeric structure and is coloured violet or brown (there
are two crystalline forms). In water, it forms a violet/green solution,
and from a slightly acid solution a hydrated solid TiCl 3 .6H 2 O
can be obtained. Hence, clearly, [Ti(H 2 O) 6 ]3+ can exist (as might
be expected since (Ti^3 ) would have a lower charge and larger radius
than (Ti4+)). The aqueous solution has reducing properties:
TiO2+(aq) + 2H 3 O+ + e~ -» Ti3+(aq) 4- 3H 2 O: E^ = +0.1 V
It must be kept under an atmosphere of nitrogen or carbon dioxide;
it reduces, for example, Fe(III) to Fe(II) and mtro-organic com-
pounds RNO 2 to amines RNH 2 (it may be used quantitatively to
estimate nitro-compounds). In neutral solution, hydrolysis occurs
to give species such as [Ti(OH)(H 2 O) 5 ]^2 , and with alkali an
insoluble substance formulated ,as Ti 2 O 3 aq' is produced; this is
rapidly oxidised in air.
Complexes of titanium(III) can be made from the trichloride—
these are either approximately octahedral, 6-coordinate (for example
TiCl 3 .3L (L = ligand) and [TiCl 2 (H 2 O) 4 ]*, formed when TiCl 3
dissolves in aqueous hydrochloric acid), or 5-coordinate with a
trigonal bipyramid structure.
Other oxidation states
Titanium forms dihalides TiX 2 , for example titanium(II) chloride,
formed by heating titanium metal and the tetrachloride to about
1200 K. TiCl 2 is a black solid, which disproportionates on standing
to TiCl 4 + Ti. Since it reduces water to hydrogen, there is no
aqueous chemistry for titanium(II). A solid oxide TiO is known.