Cobalt (III) compounds

The simple compounds of cobalt(III) are strongly oxidising:

[Co(H₂O)₆]³⁺ +e^- = [Co(H₂O)₆]^{2 +}, E⁰ = +1.92 V.

and hence the simple cobalt(III) cation cannot exist in aqueous solution (which it would oxidise to oxygen). However, the chemistry of cobalt is known for the ease of forming complexes, and for the big effect which complex formation has on relative stabilities of the + 2 and + 3 states.

As already noted, the simple salts in this oxidation state are powerful oxidizing agents and oxidise water. Since, Co(III) would also oxidise any halide except fluoride to halogen, the only simple halide salt is CoF₃, Cobalt(III) fluoride, obtained by reaction of fluorine with cobalt(II) fluoride can be used in some fluorination reactions and reacts vigorously with water.

Cobalt(III) oxide is obtained as a brown precipitate Co₂O₃ when cobalt(II) hydroxide is oxidised in alkaline conditions (or when a cobalt(III) is decomposed by aqueous alkali). On heating it gives the black mixed oxide Co₃O₄.

Hydrated cobalt(III) sulfate, Co₂(SO₄)₃^.18H₂O is obtained when cobalt(II) sulfate is oxidised electrolytically in moderately concentrated sulfuric acid solution: it is stable when dry but liberates oxygen from water. Some alums, for example KCo(SO₄)₂^.12H,O can be obtained by crystallisation from sulfuric acid solutions. In these and the sulfate, the cation [Co(H₂O)₆]³⁺ may exist; it is both acidic and strongly oxidizing.

Cobalt(III) nitrate Co(NO₃)₃ has been prepared by the reaction of dinitrogen pentoxide with cobalt(III) fluoride.

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