Question

Define primary and secondary valency of metal ions proposed by werner theory.

Answer 1

Hint: Coordination compounds retain their identity in the solution. Many theories were proposed to study about them but failed. In 1893. Werner proposed his theory called Werner’s coordination theory which explained all the properties observed in the coordination compounds. 2 valencies were explained by him which were primary and secondary valencies.

Complete answer:
-Coordination compounds are different types of compounds which exist in nature, They do not change their nature even when they are dissolved in a solution. So they are studied separately.
-Important postulates of Werner theory are
1. Central metal atoms in a coordination compound show 2 types of valencies named primary and secondary valencies.
2. Primary valency is related to the oxidation state of the compound while the secondary valency is related to the coordination number of the compound.
3. The number of secondary valencies is fixed for all the metal atoms. A particular metal can have a certain coordination number only.
4. Metal atom bonds with the ligands so as to satisfy both its primary as well as secondary valency. This differentiates it with a normal compound where the atoms satisfy only their secondary valencies.
5. Secondary valencies are responsible for a fixed shape of the coordination compounds. Shape can be decided based on coordination number as

COORDINATION NUMBER	SHAPE
2	Linear
3	Triangular
4	Tetrahedral or square planar
6	octahedral

Thus, these postulates define everything about a coordination compound and so it is important to know about both the primary as well as secondary valencies of such compounds.
-Oxidation number is the amount of charge carried by a metal ion if all the ligands and electron pairs are removed from it.

Note: Secondary valency of a metal is fixed while a metal ion can have more than 1 primary valency. Primary valency depends on the number of ligands associated with a metal ion. Primary valency is ionisable while secondary valency is non-ionisable.