A special property of transition metals is their ability to form a large number of stable coordination compounds. These compounds are formed form the reaction between two or more apparently saturated molecules capable of independent existence. For example, the white precipitate of silver chloride dissolves in aqueous ammonia or potassium cyanide. This is attributed to the formation of coordination compounds between AgCI and NH₃ or AgCI and KCN as given by equations and respectively.

AgCI + 2NH₃    →   [ Ag(NH₃)₂ ] CI
AgCI + 2KCN   →   K [ Ag(CN)₂ ] + KCI

These coordination compounds are soluble in water and they exhibit different physical and chemical properties form their reactants, i.e., form AgCI and KCN

A coordination complex may be defines as and inorganic molecule or ion that conations a central metal atom or ion surrounded by a group of atoms, ions or molecules.

Complexes play an important role both in nature and in the industrial processes. The green coloring matter present in plants is called chlorophyll. It is a coordination complex of Mg²⁺ ions. Hemoglobin which imparts red colour to our blood is porphyrin complex of Fe²⁺ ions. A large number of essential elements which are present in trace amounts in our body function as various coordination compounds. Different coordination compounds are used in industry for protection of metal surfaces, water softening. Corrosion control, electroplating, extraction of metals, pigments for paints and in soil treatment.

Coordination compounds or complexes should not be confused with double salts. Differences between coordination compounds and double salts are fundamental in nature. Some of these points are discussed below:

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