THE TRANSITION ELEMENTS 397
an acid solution of the salt with M = NH 4 (Mohr's salt, terrous
ammonium sulphate') is much less quickly oxidised by air than is
the simple iron(II) sulphate solution, and hence is used in volumetric
analysis. Iron(II)sulphide, FeS, may be prepared by heating the
elements together, or by precipitation from an iron(II) solution
by sulphide ion; it is a black solid which is non-stoichiometric, like
the oxide. The yellow sulphide FeS 2 (made up essentially of Fe^2 +
and 82 ~ ions) occurs naturally as pyrites.
COMPLEXES
As with the + 3 state, iron(II) forms a variety of complexes which
are usually 6-coordinate and octahedral. Replacement of the water
ligands in green [Fe(H 2 O) 6 ]^2 + (itself an octahedral complex) by
ammonia molecules is incomplete in aqueous ammonia, but reac-
tion of the anhydrous chloride with gaseous or liquid ammonia gives
the complex [Fe(NH 3 ) 6 ]Cl 2. The water ligands are more easily
replaced by cyanide ions to give the hexacyanoferrate(II) ion,
[Fe(CN) 6 ]^4 ~. Many salts of this ion are known, for example the
soluble yellow hydrate K 4 [Fe(CN) 6 ].3H 2 O, and the insoluble
brown copper(II) salt Cu 2 [Fe(CN) 6 ] once much used as a semi
permeable membrane in osmotic pressure determinations. The
reaction between aqueous Fe3+ ions and [Fe(CN) 6 ]^4 ~ yields an
intense blue precipitate, prussian blue, which is iron(III) hexacyano-
ferrate(II), Fe 4 [Fe(CN) 6 ] 3 ; the same material, called TurnhuWs blue.
is obtained by addition of Fe2+ (aq.) ions to [Fe(CN) 6 ]^3 " ions. The
intense colour of this compound is due to charge-transfer (p. 60). The
formation of [Fe(CN) 6 ]^4 ~ ions causes the iron(II) to change its
properties (for example it is not precipitated as the hydroxide with
alkali or as the sulphide with S^2 ~ ions); it is more readily oxidised
to the + 3 state, since
[Fe(CN) 6 ]^3 -(aq) + e~ -> [Fe(CN) 6 ]^4 (aq): E^ = + 0.36 V
When concentrated sulphuric acid is added to a nitrate in the
presence of aqueous iron(II) sulphate, the nitrogen oxide liberated
forms a brown complex [Fe(H 2 O) 5 NO]2+ which appears as a
"brown ring' at the acid-aqueous interface (test for a nitrate, p 243).
Perhaps the most important complex of iron(II) is heme (or
haeme). Haemoglobin, the iron-containing constituent of the blood,
consists essentially of a protein, globin, attached through a nitrogen
atom at one coordination position of an octahedral complex of
iron(II). Of the other five coordination positions, four (in a plane)
are occupied by nitrogen atoms, each of which is part of an organic