Modern inorganic chemistry

THE TRANSITION ELEMENTS 395

kFeO(OH)'. The a-form is used as a red pigment, as a metal polish

("jeweller's rouge') and as a catalyst.

The mixed oxide Fe 3 O 4 (tri-iron tetroxide) is a black solid, which

occurs naturally as magnetite; it is formed when iron(III) oxide is

strongly heated, and its structure is effectively made up of oxide

(O^2 ~) and iron(II) and iron(III) ions.

Iron(III) very readily forms complexes, which are commonly

6-coordinate and octahedral. The pale violet hexaaquo-ion

[Fe(H 2 O) 6 ]^3 + is only found as such in a few solid hydrated salts

(or in their acidified solutions), for example Fe 2 (SO 4 ) 3 .9H 2 O.

Fe(ClO 4 ) 3 .lOH 2 O. In many other salts, the anion may form a

complex with the iron(III) and produce a consequent colour change.

for example iron(III) chloride hydrate or solution, p. 394. Stable

anionic complexes are formed with a number of ions, for example

with ethanedioate (oxalate), C 2 O4~, and cyanide. The redox

potential of the ironll-ironlll system is altered by complex forma-

tion with each of these ligands; indeed, the hexacyanoferrate(III)

ion, [Fe(CN) 6 ]^3 ", is most readily obtained by oxidation of the corre-

sponding iron(II) complex, because

[Fe(H 2 O) 6 ]^3 + + e" -^ [Fe(H 2 O) 6 ]^2 + : E^ = + 0.77 V

[Fe(CN) 6 ]^3 - + e' -^ [Fe(CN) 6 ]^4 ~ : E^ = + 0.36 V

The thiocyanate ion SCN~ forms an intensely red-coloured complex
(most simply represented as [Fe(SCN)(H 2 O) 5 ]2+) which is a test
for iron(III). However, unlike eobalt(III), iron(III) does not form
stable hexammines in aqueous solution, although salts containing
the ion [Fe(NH 3 ) 6 ]^3 + can be obtained by dissolving anhydrous
iron(III) salts in liquid ammonia.

Oxidation state -f- 2

In this oxidation state, iron is quite readily oxidised by mild oxidising
agents, and hence in many of the reactions it is a mild reducing agent.
For acid conditions
Fe^3 + (aq) + e~ -> Fe^2 +(aq): E^ = + 0.77 V
and hence air (oxygen) will be expected to oxidise the + 2 to the +3
state. In practice, this process is usually slow, but more powerful
oxidising agents (e.g. manganate(VII) ion, dichromatefVl) ion,
hydrogen peroxide) act more rapidly and quantitatively. However
this applies strictly only to the green hexaquo-ion [Fe(H 2 O) 6 ]^2 * ;
a change to higher pH, i.e. to more alkaline conditions, changes the