Werner's Coordination Theory
The Foundation of Coordination Chemistry | Class 12
1. Introduction
Proposed by Alfred Werner in 1893 to explain the nature of bonding in coordination compounds. He was the first to propose that metals exhibit two types of valencies.
Nobel Prize: Werner was awarded the Nobel Prize in Chemistry in 1913 for this groundbreaking theory.
2. Main Postulates
Werner postulated that metal ions in coordination compounds possess two types of valency:
A. Primary Valency
- Corresponds to: Oxidation State.
- Nature: Ionizable.
- Satisfied by: Negative ions only.
- Representation: Dashed lines (- - -).
- Direction: Non-directional.
B. Secondary Valency
- Corresponds to: Coordination Number.
- Nature: Non-ionizable.
- Satisfied by: Neutral molecules or negative ions (Ligands).
- Representation: Solid lines (—).
- Direction: Directional (Determines geometry/shape).
3. Experimental Verification (Cobalt Amine Series)
Werner studied the reaction of various Cobalt(III) chloride ammines with excess Silver Nitrate ($AgNO_3$). The number of moles of $AgCl$ precipitate formed indicated the number of ionizable chloride ions (Primary valency outside the sphere).
| Werner's Formula | Modern Formula | Colour | Moles of AgCl | Total Ions |
|---|---|---|---|---|
| $CoCl_3 \cdot 6NH_3$ | $[Co(NH_3)_6]Cl_3$ | Yellow | 3 | 4 |
| $CoCl_3 \cdot 5NH_3$ | $[Co(NH_3)_5Cl]Cl_2$ | Purple | 2 | 3 |
| $CoCl_3 \cdot 4NH_3$ | $[Co(NH_3)_4Cl_2]Cl$ | Green | 1 | 2 |
| $CoCl_3 \cdot 3NH_3$ | $[Co(NH_3)_3Cl_3]$ | Violet | 0 | 0 (Neutral) |
4. Structure and Geometry
The secondary valencies are directed in space around the central metal ion, giving the complex a definite geometry.
- Coordination No. 6: Octahedral Geometry.
- Coordination No. 4: Tetrahedral or Square Planar Geometry.
Example: In $[Co(NH_3)_6]^{3+}$, the six $NH_3$ ligands are arranged octahedrally around the Cobalt ion.
5. Limitations of Werner's Theory
- It could not explain why only certain elements form coordination compounds.
- It failed to explain the magnetic and optical properties of these compounds.
- It did not explain the nature of bonding (covalent vs ionic character) between metal and ligand.
Practice Quiz
Test your knowledge on Werner's Theory.
No comments:
Post a Comment