Cobalt in Periodic Table
Cobalt (Co), chemical element, lustrous silvery-gray ferromagnetic transition metal of Group 9 (VIIIB) of the periodic table, used in making different types of alloys, and large quantities of its compounds are used in glass and ceramic industries. The hardness and tensile strength of cobalt are higher than steel.
The hexagonal crystal lattice, cobalt has chemical symbol Co, atomic number 27, atomic weight 58.94, melting point 1475°C, boiling point 2927°C, and valence shell electronic configuration [Ar] 3d7 4s2. The +2 and +3 oxidation number or state is important to derive the chemical bonding and compounds of cobalt. Co(II) is more stable than Co(III) in acid water solutions. Finely divided metallic cobalt is pyrophoric like iron but the compact metal is not attacked by air or water at ordinary temperature.
History and Occurrence
The history of the discovery of cobalt may be started with its naming. The name of the metal given from the German word cobold means evil spirit or goblin. In 1735, the Swedish chemist G. Brandt analyzed some specimens of cobold and wrote about a new semi-metal which he named cobold. Brandit describes six semi-metals like mercury, bismuth, zinc, antimony, cobalt, and arsenic. Before 1776, many scientists believed that the element was a compound of iron and arsenic. The identity of the cobalt as a new metal was established around 1780 by T Bergmann and other scientists.
Cobalt is less common among the first transition metals except for scandium. It is the thirtieth most abundant element of all known elements that occurs mainly with nickel and arsenic. The principal minerals of cobalt are arsenides and sulfides like smaltite, CoAs2, erythrite, Co3(AsO4)2. Besides arsenic, sulfur, and iron, the minerals contain 4 to 10 percent of nickel, and a varying amount of silver, copper, and lead. South Africa and Canada are the main producers of cobalt and small reserves of metal also present in Australia and Russian countries.
Properties and Compounds
Cobalt is a lustrous silvery metal with a bluish tinge. The hardness and tensile strength of the metal are greater than the steel. It is a ferromagnetic metal with a high curie temperature (1121°C). Finely divided metallic cobalt is pyrophoric like iron but compact metal is not attacked by air or water molecule at ordinary temperature. The metal is less readily dissolve in minerals acids-like nitric acid and sulfuric acid. It does not attack by aqueous alkali but readily dissolved in fused KOH at 550°C. The metal, cobalt reacts with halogens like fluorine, chlorine, bromine, and other non-metals like boron, carbon, phosphorus, sulfur on heating.
The oxidation state free energy diagram shows that the most common and important oxidation state or number of cobalt is +2 and +3. In acid solution, Co(II) is more stable than Co(III).
Compounds of Cobalt(II)
In +2 oxidation state, cobalt forms stable and simple chemical compounds, metal salts, and complexes with weak field ligands. Co(II) oxide (CoO) is formed when steam is passed over red hot metal or Co(II) carbonate, nitrate, etc. The oxide is an olive green solid stable up to 1000°C, but when heated in air it formed Co3O4. The halide like CoF2 is made by heating the double salt (CoF2, 2NH4F) or by the action of HF on CoCl2. The other halides are obtained by direct reaction of cobalt metal and halogens like chlorine, bromine, and iodine. Numerous complexes of Co(II) are known, the majority of which fall within the categories octahedral (pink) and tetrahedral (blue).
Compounds of Cobalt(III)
The chemistry of cobalt shows that the +3 oxidation state is strongly oxidizing in nature in an aqueous medium but there exists a wide range of coordination compounds with various types of ligands. The simple chemical compounds like oxides (Co2O3), fluoride (CoF3), and nitrate are also well known, which are also oxidizing in nature.
In +3 state, cobalt contains d6 configuration which gains the advantage of large crystal field stabilization energy (CFSE) with strong field ligands. Werner shows that Co(II) forms a series of amine complexes and the orange hexamine, [Co(NH3)6]+3 is readily formed by aerial oxidation of aqueous Co(II) in presence of ammonia and ammonium salts.
Oxidation state IV and V
Oxidation of aqueous Co(II) in alkaline medium by chlorine, oxygen, and ozone forms a block hydrous oxide CoO2, but this is not well characterized. Several mixed oxide species of cobalt(IV) are also reported. For example, M2CoO3 (M = K, Rb, Cs), Li8CoO8 and Ba2CoO4. The last compounds have the same structure as K2SO4 and can be prepared by heating Co(OH)2 with Ba(OH)2 or BaCO3 in oxygen at 1050°C.
Similarly, the fusion of CoO4 with NaO2 produces Na4CoO4 which contains the tetrahedral CoO4 anion with Na+ cation coordinated to oxygen atoms. A compound like Cs2CoF6 is prepared by fluorination of Cs2CoCl4 which is isomorphous with Cs2SiF6. The low oxidation state of cobalt ( 1, O, and -1) are found with strongly pi-bonding ligands like carbonyl (CO), nitrosyl (NO), and cyanogen (CN–) ligands. Cobalt metal may be detected conveniently by the deep blue color produced in borax beads which remains unchanged in reducing flame. The solution containing Co(II) produces a yellow precipitate with potassium nitrate solution in excess acetic acid medium.
Uses of Cobalt
Before the 19th century, large quantities of cobalt compounds are used in glass and ceramic industries but presently it is used in different fields like alloy or batteries making and chemical catalyst. The cobalt compounds or blue pigments are used in glass and ceramic industries due to their artistic color. Cobalt salts are also used to speed up the oxidation of oil-based paints and hence it is used as a drying agent in paints.
Nearly 30 percent of the total production of metal (about 10,000 tonnes per annum in the world) used for making paints or coloring. The almost same quantity of cobalt is used in making high-quality alloys, used in gas turbine blades or high-speed tools. Radioactive Co-60 is a beta emitter and used for the treatment of cancer. Cobalt compounds in chemistry or chemical science are also used as a chemical catalyst in the number of organic chemical reactions like oxo-process and hydrogenation reactions.