Modern inorganic chemistry

396 THE TRANSITION ELEMENTS

+ 2 species finally to insoluble kFe(OH) 2 ' (or hydrated oxide): for

this

Fe(OH) 3 + e~ -+ Fe(OH) 2 + OH'(aq): £^ = - 0.56V

and hence the reducing power is greatly increased, and Te(OH) 2 *

(white when pure) is rapidly oxidised by air. Again, replacement

of the water ligands of [Fe(H 2 O) 6 ]^2 + by other ligands will alter the

value of E^ (see below, p. 397).

THE HALIDES

The anhydrous halides FeX 2 are pale-coloured solids (FeCl 2 is

yellow) with very high melting points. The chloride may be obtained

by heating the metal in a stream of dry hydrogen chloride; it shows

some solubility in organic liquids and may be a partly cotalent solid.

However, all the halides are deliquescent, and very readily form

hydrates. Thus iron(II) chloride forms FeCl 2 .4H 2 O and FeCl 2 ,6H 2 O

(both green); in the latter, there are neutral complexes

[FeCl 2 (H 2 0) 4 ].

THE OXIDES

Iron(II) oxide FeO is prepared by heating iron(II) ethanedioate

(oxalate) in vacua:

FeC 2 O 4 -» FeO + CO + CO 2

It is a black powder, often pyrophoric, and is non-stoichiometric,

the formula Fe 0 95 O more correctly representing its average com-

position.

The 'hydroxide, Fe(OH) 2 ' has been referred to above.

OTHER IMPORTANT COMPOUNDS

Other iron(II) salts include, notably the green sulphate heptahydrate
FeSO 4. 7H 2 O which on heating yields first the white anhydrous
salt FeSO 4 and then decomposes :

2FeSO 4 -* Fe 2 O 3 + SO 2 + SO 3

Double salts of general formula M^SO 4 .FeSO 4 .6H2O (M = alkali

metal or ammonium) can be obtained by crystallisation of solutions

containing the appropriate proportions of the two simple salts: