Werner’s Theory:-

A complex should satisfy its 1° and 2° valencies. 1° valancy is oxidation state of central metal and it is satisfied by all charged species either legand or outer Ions. It is called Ionisable valency. 2° valency is co-ordination number metal. It is satisfied only by legand, either charged or neutral. It is non-ionisable valency.

Imp:- According to the defination co-ordination number is number of nearest neighbour atom not lone pair accepted. But in most of complexes both re same.

But in case of π-bonded organo- metallics both are different.

K₄ [Fe (CN)₆ ]    

1. (Oxidation State) valency of metal  = +2
2. (Coordination number) valency of metal = 6