COBALT

Atomic Number: 27
Atomic Weight: 58.93320
Electron Configuration: 1s²2s²2p⁶3s²3p⁶4s²3d⁷
Crystal Structure: Hexagonal Close-Packed
Density: 8.9 g/cm³
Melting Point: 1495 ^oC
Important Oxidation States: +2, +3

Cobalt is a hard, bright metal with a slightly bluish cast (compared to the definite bronze color of manganese metal or the red color of copper metal). As a metal, cobalt is not particularly useful. It is generally obtained from sulfide ores. The primary use of cobalt metal is as an additive to steel. Steel with useful magnetic properties is produced when cobalt and nickel are added. Cobalt metal is fairly unreactive. Like copper, it will not react with hydrochloric acid, but will react with concentrated nitric acid.

The Reaction of Cobalt Metal with Nitric Acid Cobalt metal does not react with concentrated hydrochloric acid, but will react with concentrated nitric acid as shown in this photo. The solution, originally colorless, is reddish-orange in color due to the preseence of cobalt ions in solution. The gas being formed over the surface of the liquid is nitrogen dioxide. A similar reaction occurs bewteen nitric acid and copper metal.

The two common oxidation states for cobalt are +2 and +3. In the older-style nomenclature system, the +2 ion is known as the cobaltous ion. Many cobalt(II) salts in the +2 oxidation state are commercialy available, including the chloride and nitrate. Cobalt(II) oxide is deep blue in color, and has been used to ceramic glazes and to color glass. The term "cobalt blue" refers to the blue color of cobalt oxide.

The cobalt(II) ion is labile, meaning that it exchanges ligands very rapdily. The resulting complexes may be of either octahedral or tetrahedral in geometry. In aqueous solition, cobalt(II) forms an octahedral complex ion in which it coordinated to six water molecules; this ion is pink in color. However, if an excess of chloride ion is added, the solution quickly changes to a deep blue. The deep blue color is due to a tetrachloro species, which has a tetrahedral geometry. A similar reaction occurs when thiocyanate is added, forming a magenta complex ionm, again having tetrahedral geometry

Cobalt(II) chloride hexahydrate is often used to indicate the presence of moisture. When hydrated, it is red-purple in color. Heating the solid to drive off the water, anyhdrous cobalt chloride is produced, which is blue in color. This reaction is reversible; adding water returns it to the pink hydrated form.

Cobalt Glass Cobalt glass is colored with cobalt(II) oxide, this is the origin of the term "cobalt blue"

The second common oxidation state for cobalt is +3. In the older-stytle nomenclature system, this ion is known as the cobaltic ion. Unlike the +2 ion, the +3 ion is kinetically inert, meaning that it exchanges ligands very slowly. As a result, cobalt(III) complexes have been extensively studied, and literally thousands of complexes are known. Cobalt(III) has a particulat affinity for nitrogen donor ligands, such as ammonia and ethylenediamine.

If cobalt(III) is inert, how are most compexes made? If it does not readily exchange ligands, how are the desired ligands attached? The usual technique involves oxidation of a cobalt(II) salt in the presence of the desired ligands. The oxidation can be achieved by drawing air through the solution for 60-90 minutes, or by using an oxidizing agent such as hydrogen peroxide. The complex ion that is produced is often dependent upon the reaction conditions; a few examples are listed below.


[-] Oxidation of cobalt(II) nitrate in the presence of ammonia and ammonum carbonate produces [Co(NH₃)₄CO₃]NO₃, which is burgandy in color. This is a classic laboratory preparation.

[-] Oxidation of cobalt(II) chloride in the presence of ammonia and chloride produces [Co(NH₃)₅Cl]Cl, which us mauve (pink-purple) in color

[-] The addition of activated charcoal to the above reaction produces [Co(NH₃)₆]Cl₃, which is red-orange in color

[-] Oxidation of cobalt(II) chloride with two equivalents of ethylenediamine produces trans-[Co(en)₂Cl₂]Cl, wich is green in the color. The cis form, which is blue, can be produced by dissolving the trans form in water and heating to dryness.

[-] Oxidation of cobalt(II) chloride with three equivalents of ethylenediamine produces [Co(en)₃]Cl₃, which is red-orange in color.