Explain the bonding in coordination compounds in terms of Werner’s postulates.
Solution and Explanation
(i) A metal exhibits two types of valencies namely, primary and secondary valencies. Primary valencies are satisfied by negative ions while secondary valencies are satisfied by both negative and neutral ions. (In modern terminology, the primary valency corresponds to the oxidation number of the metal ion, whereas the secondary valency refers to the coordination number of the metal ion.
(ii) A metal ion has a definite number of secondary valencies around the central atom. Also, these valencies project in a specific direction in the space assigned to the definite geometry of the coordination compound.
(iii) Primary valencies are usually ionizable, while secondary valencies are non-ionizable
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Concepts Used:
Werner’s Theory of Coordination Compounds
In 1893 Werner produced a theory to explain the structures, formation and nature of bonding in the coordination compounds. This theory is known as Werner’s theory of coordination compounds.
Postulates of Werner's Theory:
The important postulates as observed by Alfred Werner throughout his experiments are as follows:
- The complex/ coordination compounds contain a central metal atom.
- The metal atoms in a coordination compound generally show two types of valency: primary valency and secondary valency.
- The primary valencies denote the oxidation state. They are ionizable and are satisfied by the negative ions.
- Secondary valencies denote the coordination number. They are non-ionizable and are fixed for every metal atom. The secondary valency is generally satisfied by the neutral molecules or negative ions.
- The metal atoms should satisfy both primary and secondary valencies.
- The secondary valency of the atom basically shows the geometry/ polyhedra of the particular coordination compound.
Limitations of Werner’s Theory:
- Though Werner explained some properties of the coordination compound, he failed to explain the colour of the coordinate compound.
- He could not explain the magnetic and optical properties of coordination compounds.
- He could not answer the question, why does the coordination sphere have a definite geometry.