A central metal atom is surrounded by nonmetal atoms or groups of atoms in coordination compounds. These surrounding groups or atoms are called ligands, they are joined to it by chemical bonds. Examples of coordination compounds – vitamin B12, haemoglobin, and chlorophyll, dyes and igments, and catalysts used in preparing organic substances.
Coordination compounds are vastly used as catalysts (they change the rate of chemical reactions). The production of polyethylene and polypropylene are impossible without the role of certain complex metal catalysts. Organometallic coordination compounds have two molecules of an unsaturated cyclic hydrocarbon, which lacks one or more hydrogen atoms, bond on either side of a metal atom. This structure is often called the “sandwich structure”. This results in a highly stable aromatic system.
Coordination compounds in nature
Living organisms need the naturally occurring coordination compounds. Biological systems need the role of metal complexes. Many enzymes (the naturally occurring catalysts) regulating biological processes, are metal complexes (metalloenzymes); e.g., carboxypeptidase is a hydrolytic enzyme which is important in digestion, it contains a zinc ion coordinated to several amino acid residues of the protein. Another enzyme, catalase
(efficient catalyst for the decomposition of hydrogen peroxide) contains iron-porphyrin complexes. The coordinated metal ions are probably the sites of catalytic activity in both cases. Haemoglobin, which also contains iron-porphyrin complexes, can play its role as an oxygen carrier because of the iron atoms’ ability to coordinate oxygen molecules reversibly. Chlorophyll is another biologically important coordination compound, which is a
magnesium-porphyrin complex and vitamin B12 is another, a complex of cobalt with a macrocyclic ligand known as corrin.
Coordination compounds in industry
Coordination compounds are used in various different ways in chemistry and technology.
- The brilliant and intense colours of many coordination compounds, such as Prussian blue, render them of great value as dyes and pigments. Phthalocyanine complexes (e.g., copper phthalocyanine), containing large-ring ligands closely related to the porphyrins, constitute an important class of dyes for fabrics.
- Several important hydrometallurgical processes utilize metal complexes. Some metals can be extracted from their ores as ammine complexes using aqueous ammonia (Nickel, cobalt, and copper). Differences in the stabilities and solubilities of the ammine complexes can be utilized in selective precipitation procedures that bring about separation of the metals. The purification of nickel can be done by reaction with carbon monoxide to form the volatile tetracarbonyl-nickel complex, which can be distilled and thermally decomposed to deposit the pure metal. Aqueous cyanide solutions usually are employed to separate gold from its ores in the form of the extremely stable dicyanoaurate (−1) complex. Cyanide complexes also find application in electroplating.
History of coordination compounds
The bright red alizarin dye first used in India and known to the ancient Persians and Egyptians is perhaps the earliest known coordination compound (a calcium aluminium chelate complex of hydroxyanthraquinone). German chemist, physician, and alchemist Andreas Libavius’s description in 1597 of the blue colour (due to [Cu(NH3)4]2+) formed when lime water containing sal ammoniac (NH4Cl) comes into contact with brass, is the first scientifically recorded observation of a completely inorganic coordination compound.