Coordination Compound

A coordination compound is any of a class of substances having chemical structures in which a central metal atom is surrounded either by groups of atoms or nonmetal atoms, which are called ligands, joined to it by the chemical bonds. Substances such as haemoglobin, chlorophyll, vitamin B12, pigments and colourants, and catalysts that are included in the preparation of organic substances provide coordination compounds.

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About Coordination Compounds

A primary application of coordination compounds is their use as catalysts that serve to alter the chemical reaction’s rate. For example, Certain complex metal catalysts play a key role in polypropylene and polyethylene production. In addition, a very stable class of organometallic coordination compounds has provided the impetus to organometallic chemistry development. Occasionally, "sandwich" configurations describe organometallic coordination compounds, where two molecules of unsaturated cyclic hydrocarbon, missing one or more atoms of hydrogen, bond to either side of a metal atom. This results in an aromatic system, which is highly stable.

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Structure of Coordination Compounds

Werner has originally postulated that the coordination compounds are formed due to the reason the central atoms carry the capacity to produce coordinate or secondary bonds, in addition to the valence or normal bonds. A complete description of the coordinate bonding, in terms of electron pairs, in the 1920s, has become possible, following the concept’s introduction that all the covalent bonds consist of electron pairs shared between atoms. It is an idea chiefly advanced by the American physical chemist named Gilbert N. Lewis. In this Lewis’s formulation, when both electrons are contributed by one of the atoms, similar to the boron-nitrogen bond formed when the substance boron trifluoride (BF3) combines with ammonia, the bond is referred to as a coordinate bond:

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Coordination Compounds in Nature

The coordination compounds, which occur naturally, are vital to living organisms. And, metal complexes play a variety of essential roles in biological systems. Several enzymes, the naturally occurring catalysts, which regulate biological processes, are the metal complexes (otherwise called metalloenzymes). For example, carboxypeptidase, which is a hydrolytic enzyme essential in the digestion process, contains a zinc ion, coordinated too many amino acid residues of the protein. Another enzyme, catalase, an efficient catalyst for hydrogen peroxide decomposition, contains iron-porphyrin complexes. In both cases, the coordinated metal ions are probably the catalytic activity sites. Also, haemoglobin contains iron-porphyrin complexes, where its role as an oxygen carrier is related to the iron atom’s ability to coordinate oxygen molecules reversibly. Other biologically essential coordination compounds are given as vitamin B12, a cobalt complex with a macrocyclic ligand called corrin, and chlorophyll (a magnesium-porphyrin complex).

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Coordination Compounds in Industry

The coordination compound applications in chemistry and technology are varied and several. The intense and brilliant colours of several coordination compounds, like Prussian blue, render them of great value as pigments and dyes. An important class of dyes for fabrics is phthalocyanine complexes (for example, copper phthalocyanine) containing large-ring ligands, which are closely linked to porphyrins.

Many important hydrometallurgical processes utilize metal complexes. Cobalt, copper, and nickel can be extracted from their ores as ammine complexes with aqueous ammonia. Differences in the solubilities and stabilities of the ammine complexes are utilized in selective precipitation procedures, which bring about a metal separation. The nickel purification can be affected by the reaction with carbon monoxide to produce the volatile tetra-carbonyl nickel complex, which can be distilled and thermally decomposed for pure metal deposit. In the form of the extremely stable dicyanoaurate (-1) complex, aqueous cyanide solutions are generally used in the separation of gold from its ores. Cyanide complexes are also finding applications for electroplating.

Characteristics of Coordination Compounds

Coordination compounds have been extensively studied due to the reason they reveal chemical bonding and molecular structure, too, as because of the useful properties and unusual chemical nature of certain coordination compounds. The complexes or general class of coordination compounds, as they are sometimes called, are diverse and extensive. The substances in the class can be composed of electrically neutral molecules or of positively or negatively charged species (which are ions).

Among several coordination compounds having neutral molecules is uranium hexafluoride (UF₆) or uranium(+6) fluoride. The structural formula of the compound indicates the actual arrangement of the atoms in molecules:

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In the formula given above, the solid lines, which represent bonds between atoms, depicts that four of the fluorine (F) atoms are bonded to the single uranium (U) atom and lie in a plane with it, where the plane is depicted by dotted lines (the one, which does not represent bonds), whereas the other two fluorine atoms (which can also be bonded to the uranium atom) that lie above and below the plane, respectively.

Example of Coordination Compounds

Some examples of the ionic coordination complex are given as a hydrated ion of Hexa-aqua nickel(2+) ion, nickel, (Ni), [Ni(H2O)₆]2+, where the structure is shown below. In this structure, the lines and symbols are used as above, and the brackets and the “two plus” (2+) sign represents that the double positive charge has been assigned to the unit as a whole.

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