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The theory of coordination compounds by Werner describes the formation and structure of complex compounds. It is called Werner’s Theory of Coordinate Compounds.

The most important postulates of  Werner’s Theory of Coordinate Compounds are:

• In coordination compounds, the central metal or metal atoms exhibit two types of valency, the primary valency and the secondary valency. The primary valency corresponds to oxidation state and the secondary valency corresponds to coordinate number.

• Every metal atom has a fixed number of secondary valencies, so, it has fixed coordinate number.

• The metal atom tends to satisfy both its primary and secondary valancies. Primary valency is satisfied by negative ion whereas secondary valancies are satisfied by negative ion or by neutral molecules.

• The secondary valancies are always directed towards fixed position in space and this cause definite geometry of the coordinate compound. If a metal ion has six secondary valencies, these are arranged octahedrally around the central metal ion. If the metal ion has four secondary valencies, these are arranged in either tetrahedral or square planar arrangement around the central metal ion. The secondary valency thus determines the stereochemistry of the complex ion. On the other hand, the primary valency is non-directional.

In chemistry, coordination number defined by Werner is the total number of neighbors of a central atom in a molecule or ion.

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