COBALT COMPLEXES 237

Reactions

Cobalt(II) chloride undergoes many double decomposition reactions in aqueous solution to produce precipitates of insoluble cobalt salts. For example, heating its solution with sodium carbonate yields cobalt(II) carbonate:

Reaction with alkali hydroxide produces cobalt(II) hydroxide:

CoCl2 + 2NaOH → Co(OH)2 + 2NaCl

Reaction with ammonium hydrogen phosphate yields cobalt(II) phosphate:

3CoCl2 + 2(NH4)2HPO4 → Co3(PO4)2 +4NH4Cl + 2HCl

While cobalt(II) fluoride is the product of the reaction of anhydrous cobalt(II) chloride with hydrofluoric acid, cobalt(III) fluoride is obtained from fluorination of an aqueous solution of cobalt(II) chloride.

Addition of potassium nitrite, KNO2 to a solution of cobalt(II) chloride yields yellow crystalline potassium hexanitrocobaltate(III), K3Co(NO2)6.

Analysis

Elemental composition: Co 45.39%, Cl 54.61%. Aqueous solution of the salt or acid extract may be analyzed for cobalt by AA, ICP, or other instrumental techniques following appropriate dilution. Chloride anion in the aqueous solution may be measured by titration with silver nitrate using potassium chromate indicator, or by ion chromatography, or chloride ion-selective electrode.

Toxicity

The compound is toxic at high doses. Symptoms include chest pain, cutaneous flushing, nausea, vomiting, nerve deafness, and congestive heart failure. The systemic effects in humans from ingestion include anorexia, increased thyroid size, and weight loss (Lewis (Sr.), R. J. 1996. Sax’s Dangerous Properties of Industrial Materials, 9th ed. New York: Van Nostrand Reinhold). Ingestion of a large amount (30–50 g) could be fatal to children.

COBALT COMPLEXES

Cobalt forms many complexes in both the divalent and trivalent states. While the d7Co2+ ion exhibits a coordination number of four or six in the trivalent state, the d6Co3+ ion mostly exhibits coordination number six. Also, trivalent cobalt forms more stable complexes than Co2+ ion, and there are many more of them. The most common donor atom in cobalt complexes is nitrogen,

238 COBALT COMPLEXES

having ammonia and amines as ligands forming numerous complexes. Many cobalt cyanide complexes are known in which CN– coordinates to the cobalt ion through the carbon atom. In aquo complexes, water molecules coordinate through the oxygen atom. Sulfur ligands and halide ions also form numerous complexes with both Co2+ and Co3+ ions.

Cobalt complexes have limited but some notable applications. Pentacyanocobalt(II) ion can activate molecular hydrogen homogeneously in solution and therefore can act as a hydrogenation catalyst for conjugated alkenes. Cobalt ammine chelates exhibit catalytic behavior in hydrolysis of carboxylate esters, phosphate esters, amides, and nitriles. Single crystals of cyanide complex are used in laser studies. Many aquo-halo mixed complexes are used in making invisible or sympathetic inks and color indicators for desiccants. Certain chelators, such as cobalt ethylenediamine complexes, have unusual oxygen-carrying properties. These polyfunctional donor molecules have the ability to readily absorb and release oxygen. They are used as a convenient source of purified oxygen.

Cobalt(II) forms more tetrahedral complexes than any other transition metal ion. Also, because of small energy differences between the tetrahedral and octahedral complexes, often the same ligand forms both types of Co(II) complexes in equilibrium in solutions.

Some examples of Co2+ complexes having varying coordination number and geometry, are presented below: