Hexamminecobalt(II1) salts may be prepared by any of three methods that depend on oxidation of cobalt(I1) ion in ammoniacal solution : (1) air
oxidation, with formation of the pentammine ion, which is converted to-the hexammine by heating with aqueous ammonia under pressure;
(2) oxidation with an agent such as hydrogen peroxide, iodine, potassium permanganate, lead dioxideo,or hypochlorite solutions;
(3) oxidation in the presence of a catalyst that allows equilibrium between the pentammine and hexammine ions to be established at room temperature
and atmospheric pressure.
These compounds may also be prepared indirectly from other hexamminecobalt(II1) salts.
In the best of the catalytic methods diamminesilver ion or decolorizing charcoal is used as a catalyst. The method devised by J. Bjerruml* in which
decolorizing charcoal is the catalyst is simple, gives high yields of pure product, and is not time-consuming. A high concentration of ammonium salt
is sufficient to stabilize the hexamminecobalt(III) ion, and the carbon serves only to establish the equilibrium. Air is used as oxidant except when
the cobalt(I1) compound is slightly soluble in the ammoniacal solution, as in the preparation of [CO(NH3)6]Br3 for which hydrogen peroxide is
preferable. |
