Question

What are primary valencies and secondary valencies? Give an example?

Answer 1

This concept is from the coordination compounds. Also, these primary valencies and secondary valencies are the postulates of Werner’s theory. Primary valency helps in finding oxidation state and charge and secondary valency helps in finding coordination number of the complex.

Complete step by step answer:
As we know, the primary and secondary valencies are the postulates of Werner’s theory. According to Werner’s theory, the central metal in coordination compounds show two types of valencies. They are primary valency and secondary valency.
The primary valency in coordination compounds is the number of negative ions which are equivalent to the charge on the metal ion. The secondary valency is the number of ions of molecules that are coordinated to the metal ion or it is the number of ligands attached or coordinated to metal ions.
For example, consider the coordination compound${\text{CoC}}{{\text{l}}_{\text{3}}}{\text{.6N}}{{\text{H}}_{\text{3}}}$: In this compound, the coordination number of cobalt is 6 and ${\text{N}}{{\text{H}}_{\text{3}}}$satisfy all the 6 secondary valencies. Chloride ions satisfy the 3 primary valencies. These are non-directional in character. These chlorides will precipitate instantaneously on the addition of silver nitrate. The total number ions, in this case, is 4, three chloride ions and one complex ion.

Additional Information: The other important postulates of Werner’s theory:

• The primary valency relates to the oxidation state and secondary valency relates to the coordinate number.
• The number of secondary valencies is fixed for every metal atom. It means that the coordination number is fixed.
• 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.

Note: You need to know a thorough understanding about primary valency and secondary valency. Remember that primary valency is ionisable and the secondary valencies are non-ionisable in the complex. The central atom is surrounded by ligands and is projected in space giving specific shape to the complex.