The preparation of Co(III) pentaammine complexes is carried out most conveniently starting from CoCl2·6H2O. The first step in the syntheses of these complexes requires oxidation to convert the metal from the +2 to the +3 oxidation state. The oxidation reaction is best carried out using concentrated H2O2 in basic ammonia solution. Hydrogen peroxide as an oxidizing agent generates only OH– as reduced product, and the presence of ammonia guarantees that
the Co3+ will be stabilized immediately by formation of amine complex(es). (The Co3+ ion is not stable in aqueous media since it forms the strongly oxidizing hexaaqua complex.)