What is the theory that explains the coordination compound?

 Theory that explains the coordination compounds

Werner proposed a theory which tells us about the coordination compounds. The postulates of this theory are as follows:

1.         Every element exhibits two types of valencies:

            (a)        Primary Valency

            (b)        Secondary Valency

In modern terminology, primary valency corresponds to the oxidation state of the central metal and secondary valency is satisfied by negative ions. This is also called principal, ionisable or ionic valency. Its attachment to the metal is shown by dotted lines.

The secondary valency is satisfied by neutral molecules or negative ions. This valency is non – ionic or non – ionisable. The donor atoms of the ligands which satisfy the coordination number are directly attached to the metal atom and shown by thick lines.

2.         In a coordination compound, there are two spheres of attraction. One is the coordination sphere, where the central atom is coordinated to atoms or groups. This refers to the complex ion compound.

3.         Every element tends to satisfy both its primary and secondary valencies. A negative ion when present in the coordination sphere shows a dual behavior. It may satisfy both primary and secondary valencies. The presence of negative ion in the coordination sphere reduces the charge on the negative ions present in the coordination sphere are not ionized. Thus, the following coordination number remains the same.

4.         The ligands which satisfy the secondary valency are directed towards fixed position in the space. The geometry of the complex ion depends on the coordination number. If the metal has coordination number 6, the complex is octahedral, i.e., six position around the metal are occupied by six donor atoms of the ligands octahedrally. On the other hand, if the coordination number is 4, the geometry of the complex may be tetrahedral or square planar. This postulate predicted the existence of different types of isomerism in coordination compounds.