1. In complex ion/compound metal atoms exhibit two types of valencies. These are :
(a) Primary valency, (b) Secondary valency.
The primary valency is ionisable, whereas secondary valency is non-ionisable. In modern terminology primary valency corresponds to oxidation state and the secondary valency corresponds to coordination number.
2. Every metal ion/atom has a fixed number of secondary valencies i.e. it has a fixed coordination number.
3. The metal atom/ion 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 ions or neutral molecules. In certain cases, a negative ion may satisfy both type of valencies.
4. The secondary valencies are always directed towards the fixed positions in space and thereby giving a definite geometry to the complex. For example, if a metal ion has six secondary valencies, these are arranged octahedrally around 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. Thus, secondary valencies determines the stereochemistry of the complex. On the other hand primary valencies are non-directional.