Question

Discuss Werner’s Theory of coordination compounds.

Hint:

Werner’s Theory distinguishes primary (ionizable) and secondary (non-ionizable) valencies and explains coordination number and geometry of complexes.

Answer 1

Concept: Werner’s Theory, proposed by Alfred Werner in 1893, laid the foundation of modern coordination chemistry. It explained the structure, bonding, and properties of coordination compounds that earlier theories failed to describe.
Step 1: Main postulates of Werner’s Theory. Werner proposed that metals in coordination compounds exhibit two types of valencies:

• 
  Primary valency (ionizable valency)
• 
  Secondary valency (non-ionizable valency)

Step 2: Primary valency.

• Corresponds to the oxidation state of the metal
• Satisfied by negative ions
• Ionizable and can be detected in solution
• Example: Cl$^-$ ions outside the coordination sphere

Step 3: Secondary valency.

• Corresponds to the coordination number of the metal
• Satisfied by ligands (neutral molecules or anions)
• Non-ionizable and remain within the coordination sphere
• Responsible for geometry of the complex

Step 4: Spatial arrangement of ligands. Werner suggested that ligands attached through secondary valencies occupy fixed positions in space, giving definite geometries such as:

• Octahedral (coordination number 6)
• Square planar (coordination number 4)
• Tetrahedral (coordination number 4)

Step 5: Example. In $\text{CoCl}_3 \cdot 6\text{NH}_3$:

• Three Cl$^-$ satisfy primary valency
• Six NH$_3$ molecules satisfy secondary valency

Significance: Werner’s Theory successfully explained:

• Ionization behavior of complexes
• Existence of coordination numbers
• Isomerism in coordination compounds
• Geometry of complexes

Conclusion: Werner’s Theory revolutionized coordination chemistry by introducing the concepts of primary and secondary valencies and explaining the structure and bonding of coordination compounds.