Question

The co-ordination number of a metal in co-ordination compounds is the:

• A. same as secondary valency
• B. sum of primary and secondary valencies
• C. same as primary valency
• D. difference between secondary and primary valencies

Answer 1

The correct option is A same as secondary valency

Some of the postulates of Werner’s theory are:

1. The central metal atom in coordination compounds show two types of valency. They are the primary and the secondary valency.
2. The primary valency relates to the oxidation state and the secondary valency relates to the coordinate number.
3. The number of secondary valences is fixed for every metal atom. It means that the coordination number is fixed.
4. The metal atom works towards satisfying both its primary and secondary valencies. A negative ion satisfies the primary valency. On the other hand, a negative ion or neutral molecules satisfy secondary valencies.
5. The secondary valencies point towards a fixed position in space. This is the reason behind the definite geometry of the coordinate compound. For example, let us consider the case of a metal ion having six secondary valencies. These arrange octahedrally around the central metal ion. If the metal ion has four secondary valencies, these arrange in either tetrahedral or square planar arrangement around the central metal ion. Therefore, we see that the secondary valency determines the stereochemistry of the complex ion. On the other hand, the primary valency is non-directional.