Cobalt (II) compounds

Salts. In some respects these salts resemble those of iron; the aquo-cation [Co(H₂O)₆]²⁺ (pink) occurs in solution and in some solid salts, for example CoSO₄^.7H₂O. However, this aquo cation is less strongly reduced than [Fe(H₂O)₆]²⁺ and the water ligands are more readily replaced by other ligands than for iron(II) (see below). [Co(H₂O)₆]²⁺ is only slightly acid and a normal, hydrated carbonate CoCO₃^. 6H₂O can be precipitated by the 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 through direct combination of the elements, or through dehydration of their hydrates. Anhydrous cobalt(II) chloride is blue, and the solid contains octahedrally-coordinated cobalt; the hydrated salt CoCl₂^. 6H₂O 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 cobalt(II) ions. The precipitate is often blue, but becomes pink on standing; it dissolves in excess alkali giving the blue [Co(OH)₄]^2– ion, and in slightly alkaline solution is easily oxidized by air to a brown solid of composition CoO(OH).

Cobalt(II) sulfide is precipitated as a black solid due to addition of sulfide ion to a solution of a cobalt(II) salt in alkaline solution.

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