394 THE TRANSITION ELEMENTS
resemblance to manganese(II), except that most iron(III) compounds
show high paramagnetism, i.e. the electrons remain unpaired.Iron(III) chloride is a black, essentially covalent solid, in which each
iron atom is surrounded octahedrally by six chlorine atoms. It is
prepared by direct combination of iron with chlorine or by dehydra-
tion of the hydrated chloride, by one of the methods given on p. 343).
When the anhydrous solid is heated, it vaporises to form first
Fe 2 Cl 6 molecules, then the monomer FeCl 3 and finally FeCl 2 and
chlorine. It fumes in air (with hydrolysis) and dissolves readily in
water to give a yellow (dilute) or brown (concentrated) solution,
which is strongly acidic. Crystallisation gives the yellow hydrate
FeCl 3 .6H 2 O which has the structure [FeCl 2 (H 2 O) 4 ]CL2H 2 O, i.e.
contains the octahedral complex ion [FeCl 2 (H 2 O) 4 ] + ; ions of this
general type are responsible for the colours of the aqueous solution
of iron(III) chloride. In the presence of excess chloride *lon, both
tetrahedral [FeQ 4 ]~ and octahedral [FeCl 6 ]^3 ~ can be formed.
Iron(III) chloride forms numerous addition compounds, especially
with organic molecules which contain donor atoms, for example
ethers, alcohols, aldehydes, ketones and amines. Anhydrous iron(III)
chloride is soluble in, for example, ether, and can be extracted into
this solvent from water; the extraction is more effective in presence
of chloride ion. Of other iron(III) halides, iron(III) bromide and
iron(III) iodide decompose rather readily into the +2 halide and
halogen.
IRON(III) OXIDES AND HYDROXIDE
If an aqueous solution of an iron(III) salt is treated with alkali, a
red-brown precipitate of Iron(III) hydroxide' is obtained; this is
probably best represented as FeO(OH). On strong heating it gives
the red oxide Fe 2 O 3. Iron(III) oxide, Fe 2 O 3 , occurs naturally as
haematite, and can also be prepared by strong heating of iron(II)
sulphate :
2FeSO 4 -» Fe 2 O 3 + SO 2 + SO.,It shows some amphoteric behaviour, since it dissolves in alkali
(concentrated aqueous or fused) to give a ferrate(III) ; the equation
may be written as
FeO + 2OH"Iron(II) oxide exists in two forms, the red a-form (paramagnetic)
and the y-form (ferromagnetic) obtained by careful heating of