Modern inorganic chemistry

THE TRANSITION ELEMENTS 367

water ligands can be partly or completely displaced by addition of

other ligands such as ammonia or chloride ion.The factors which

govern the displacement of one ligand by another are rather com-

plicated ; thus, for example, ammonia will often replace water as a

ligand, to form ammonia-metal complexes, but this does not happen

readily with all transition metal ions (notably not with Fe2+, Fe^3 +

and Mn2+). However, most complexes of metal ions in oxidation

states 2 or 3 are prepared by displacement of the water by other

ligands, for example NH 3 , CN~, halide". Complexes with metal

oxidation states 0 are not easily prepared in solution; the metal

carbonyls can, however, be prepared by direct reaction, e.g.

Ni + 4CO -> Ni(CO) 4

finely

divided

metal

For complexes with high metal oxidation states, special methods

are used, since these complexes can only exist with certain ligands

(see above).

Some properties of complex metal ions in aqueous solution

In an aquo-complex, loss of protons from the coordinated water

molecules can occur, as with hydrated non-transition metal ions

(p. 45). To prevent proton loss by aquo complexes, therefore, acid

must usually be added. It is for these conditions that redox potentials

in Chapter 4 are usually quoted. Thus, in acid solutions, we have

[Fe(H 2 0) 6 ]^3 + + <T -> [Fe(H 2 0) 6 ]^2 + : E* =' + 0.77 V

In the absence of acid, the half-reaction will approximate to:

[Fe(H 2 O) 5 (OH)]^2 + + e~ -» [Fe(H 2 O) 5 (OH)] +

for which E^ is indeterminate, but certainly less than E^ in acid
solution. In presence of alkali, the half-reaction becomes, effectively,

[Fe(H 2 O) 3 (OH) 3 ] + H 2 O + e~ -> [Fe(H 2 O) 4 (OH) 2 ] + OH'

for which E^ = - 0.56 V. Hence the less acidic a solution containing

Fe(II) is, the more easily is it oxidised and solutions of iron(II)

salts must be acidified to prevent oxidation by air. A more impres-

sive demonstration of the effect of change of ligand on oxidation-

reduction behaviour is provided by the following scheme: