404 THE TRANSITION ELEMENTS
example CoSO 4 .7H 2 O (cf. FeSO 4 .7H 2 O). However, this aquo
cation is less strongly reducing than [Fe(H 2 O) 6 ]^2 \ and the water
ligands are more readily replaced by other ligands than for iron(II)
(see below). [Co(H 2 O) 6 ]2+ is only slightly acid and a normal,
hydrated carbonate CoCO 3. 6H 2 O can be precipitated by addition
of carbonate ion to a simple cobalt(II) salt provided that an
atmosphere of carbon dioxide is maintained over the solution.
Cobalt(II) halides can be obtained by direct combination of the
elements, or by dehydration of their hydrates. Anhydrous cobalt(II)
chloride is blue, and the solid contains octahedrally-coordinated
cobalt; the hydrated salt CoCl 2. 6H 2 O is pink, with each cobalt
surrounded by four water molecules and two chloride ions in a
distorted octahedron.
Cobalt(II) hydroxide is obtained as a precipitate when hydroxide
ion is added to a solution containing eobalt(II) ions. The precipitate
is often blue, but becomes pink on standing; it dissolves in excess
alkali to give the blue [Co(OH) 4 ]^2 ~ ion, and in slightly alkaline
solution is easily oxidised by air to a brown solid of composition
ComO(OH).Cobalt(II) sulphide is precipitated as a black solid by addition of
sulphide ion to a solution of a cobalt(II) salt in alkaline solution.COMPLEXES
These are of two general kinds: octahedral, pink complexes and
tetrahedral: blue complexes. If cobalt(II) chloride is dissolved in
aqueous solution, the predominant species is the hexaaquo-ion
[Co(H 2 O) 6 ]2+ (pink). If this solution is heated, it becomes blue, and
the same effect is observed if chloride ion is added in excess. This
colour change is associated with the change
[Co(H 2 O) 6 ]^2 +
pink, H 2 U blue,
octahedral tetrahedralbut ions intermediate between these two species can also exist in
the solution. None of these species can be oxidised to cobalt(III) in
aqueous solution; but if ammonia is added to the pink solution
containing the hexaaquo-ion, the water ligands are displaced by
ammonia and the hexammino-ion [Co(NH 3 ) 6 ]2+ is formed; this is