Question

Explain Werner’s postulates related to the bonding in coordination compounds.

Hint:

Remember: Primary valencies = oxidation state (ionizable), Secondary valencies = coordination number (non-ionizable).

Answer 1

Step 1: Introduction.
Alfred Werner, in 1893, proposed a theory to explain the structure and bonding in coordination compounds. His postulates laid the foundation of coordination chemistry.
Step 2: Werner’s Postulates.
1.
Dual valencies:
Every metal atom exhibits two types of valencies: -
Primary valency (ionizable): These correspond to the oxidation state of the metal and are satisfied by negative ions. They are ionizable and represented by dotted lines.
-
Secondary valency (non-ionizable): These correspond to the coordination number and are satisfied by ligands. They are non-ionizable and represented by solid lines.
2.
Fixed coordination number:
Each metal has a fixed number of secondary valencies (coordination number). For example, Co(III) has coordination number 6.
3.
Geometry of complexes:
The secondary valencies are directed towards fixed positions in space, leading to definite geometrical shapes (octahedral, tetrahedral, square planar, etc.).
4.
Satisfaction of both valencies:
A complex compound is formed when both primary and secondary valencies of a metal are satisfied. Example: \([Co(NH_3)_6]Cl_3\). Here, Co has: - Primary valency = 3 (satisfied by 3Cl\(^-\))
- Secondary valency = 6 (satisfied by 6 NH\(_3\)) Conclusion:
Werner’s postulates explain the ionizable and non-ionizable nature of ligands, coordination number, and geometry of coordination compounds, forming the basis of modern coordination chemistry.