1. The central metal ion in any complex ion/compound exhibits two types of valencies, these are
(a) Primary valency
(b) Secondary valency.
2. The primary valency is ionisable and corresponds to the oxidation state of the metal joining the central ion.
3. The secondary valency is non – ionisable. Every central ion has a fixed number of secondary valencies. This number is called the coordination number of the central ion.
4. The primary valency of the metal ion is always satisfied by a negative ion. The attachment of the central metal ion to the negative ligand is shown by dotted lines.
5. The secondary valencies are satisfied by either negative ions or neutral molecules. The secondary valencies are shown by their lines. The molecules or ions that satisfy secondary valency are called ligands.
6. The ligands which satisfy the secondary valencies must point out in the definite directions in space. Whereas the primary valencies are non – directional in nature. The spatial arrangement of the secondary valencies around the central metal ion is called coordination polyhedron.
7. The secondary valencies are responsible for isomerism in the coordination compounds,
8. Werner’s representation of [CO(NH3)6]Cl3
