In order to explain the nature of bonding & structure of coordination compounds,Werner in 1893 proposed a theory called as Werner’s Coordination Theory.The postulates of theory are as follows:

1.In coordination compounds,metal atoms exhibit 2 types of valencies namely, primary & secondary valency. The primary valency is ionizable whereas the secondary valency is non – ionizable, i.e primary valency corresponds to oxidation state & secondary valency corresponds to coordination number.

2.Every metal atom has a fixed number of secondary valencies i.e fixed coordination number.

3.The metal atom tends to satisfy both its primary as well as secondary valencies.Primary valencies are satisfied by negative ions whereas secondary valencies are satisfied by negative or by neutral molecules.

4.The secondary valencies are always directed towards the fixed positions in space & this leads to defininte geometry of the coordination compound.For eg if the metal ion has four secondary valencies, these are arranged in either tetrahedral or square planar arrangement around the central metla ion. The secondary valencies, thus, determine the stereochemistry of the complex