Werner's Theory and CoCl₃·6NH₃
According to Werner's Coordination Theory, a metal ion in a coordination compound possesses two types of valencies:
Primary Valency
Corresponds to oxidation state.
Usually ionisable.
Secondary Valency
Corresponds to coordination number.
Non-ionisable.
Alfred Werner proposed the coordination theory in 1893 to explain the structure and behaviour of coordination compounds.
Important Postulates
- A metal ion exhibits two types of valencies: primary and secondary.
- Primary valency corresponds to the oxidation state of the metal and is generally satisfied by negative ions.
- Secondary valency corresponds to the coordination number of the metal ion.
- Secondary valencies are directed in space and determine the geometry of the coordination entity.
- Primary valencies are generally ionisable, whereas secondary valencies are non-ionisable.
Secondary Valency → Coordination Number
Primary Valency
Primary valency represents the oxidation state of the central metal ion.
For cobalt in CoCl₃·6NH₃:
Therefore, cobalt has a primary valency of 3.
The primary valency is satisfied by three chloride ions:
Secondary Valency
Secondary valency represents the coordination number of the central metal ion.
In CoCl₃·6NH₃, all six NH₃ molecules are directly attached to the cobalt ion.
Therefore, the secondary valency of cobalt is 6.
The six NH₃ molecules satisfy the six secondary valencies.
Satisfied by 6 NH₃ ligands
Coordination Entity
↓
[Co(NH3)6]Cl3
The correct Werner representation is:
Inside Coordination Sphere
Six NH₃ molecules are coordinated directly to Co³⁺.
Outside Coordination Sphere
Three chloride ions are outside the coordination sphere and are ionisable.
Primary Valency = 3
Secondary Valency = 6
When dissolved in water, the compound ionises as:
Thus, it gives four ions in solution:
| Feature | Primary Valency | Secondary Valency |
|---|---|---|
| Meaning | Oxidation state | Coordination number |
| Nature | Generally ionisable | Non-ionisable |
| In CoCl₃·6NH₃ | 3 | 6 |
| Satisfied by | 3Cl⁻ | 6NH₃ |
| Position | Outside coordination sphere | Inside coordination sphere |
The secondary valency of cobalt is 6. Therefore, six NH₃ ligands are arranged around Co³⁺.
Geometry = Octahedral
Thus, the coordination entity [Co(NH₃)₆]³⁺ has an octahedral arrangement.
Q. What are the primary and secondary valencies of Co in CoCl₃·6NH₃?
Answer:
Cobalt is in the +3 oxidation state. Therefore:
Six NH₃ molecules are directly coordinated to cobalt:
Q. Explain primary and secondary valencies using CoCl₃·6NH₃.
CoCl₃·6NH₃ is represented as:
Co is in +3 oxidation state, so its primary valency is 3. It is satisfied by three ionisable Cl⁻ ions outside the coordination sphere.
Six NH₃ molecules are directly bonded to Co³⁺, so the secondary valency is 6.
Q. Explain the structure of CoCl₃·6NH₃ according to Werner's Theory.
According to Werner's theory, cobalt exhibits primary and secondary valencies.
The compound is represented as:
Primary valency: Co is in +3 oxidation state. Thus primary valency = 3, satisfied by three Cl⁻ ions outside the coordination sphere.
Secondary valency: Six NH₃ molecules are directly attached to Co³⁺. Therefore, secondary valency = 6.
Secondary Valency = 6
Coordination Number = 6
Geometry = Octahedral
Q. Apply Werner's Theory to explain primary and secondary valencies in CoCl₃·6NH₃.
Primary Valency
Cobalt has oxidation state +3. Hence its primary valency is 3. It is satisfied by three chloride ions outside the coordination sphere. These chloride ions are ionisable.
Secondary Valency
Six NH₃ molecules are directly attached to Co³⁺. Hence the secondary valency is 6. It represents the coordination number.
Representation
Ionisation
Secondary Valency = 6
Coordination Number = 6
Q. State Werner's Theory and use it to explain the primary and secondary valencies in CoCl₃·6NH₃.
1. Werner's Theory
According to Werner, a metal ion in a coordination compound possesses two types of valencies: primary and secondary.
2. Primary Valency
Primary valency corresponds to the oxidation state of the metal. For cobalt:
Primary Valency = 3
Three Cl⁻ ions satisfy the primary valency. They remain outside the coordination sphere and are ionisable.
3. Secondary Valency
Secondary valency corresponds to the coordination number. Six NH₃ molecules are directly bonded to Co³⁺.
4. Structural Formula
5. Ionisation
Therefore, the compound gives four ions in aqueous solution: one complex cation and three chloride ions.
6. Geometry
Since the coordination number is 6, the geometry of [Co(NH₃)₆]³⁺ is octahedral.
Primary Valency = 3 → satisfied by 3Cl⁻
Secondary Valency = 6 → satisfied by 6NH₃
Coordination Number = 6
Complex = [Co(NH₃)₆]Cl₃
Geometry = Octahedral
1. Werner's Theory was proposed by:
✅ Answer
B) Alfred Werner
2. According to Werner, a metal ion has:
✅ Answer
B
3. Primary valency corresponds to:
✅ Answer
B) Oxidation state
4. Secondary valency corresponds to:
✅ Answer
B) Coordination number
5. Primary valency is generally:
✅ Answer
B) Ionisable
6. Secondary valency is generally:
✅ Answer
B) Non-ionisable
7. In CoCl₃·6NH₃, the oxidation state of Co is:
✅ Answer
C) +3
8. The primary valency of Co in CoCl₃·6NH₃ is:
✅ Answer
B) 3
9. The secondary valency of Co in CoCl₃·6NH₃ is:
✅ Answer
C) 6
10. The coordination number of Co in [Co(NH₃)₆]Cl₃ is:
✅ Answer
C) 6
11. In [Co(NH₃)₆]Cl₃, the chloride ions are:
✅ Answer
B
12. The ligands directly attached to Co in [Co(NH₃)₆]Cl₃ are:
✅ Answer
B) NH₃
13. [Co(NH₃)₆]Cl₃ gives how many ions in aqueous solution?
✅ Answer
C) 4
14. The geometry of [Co(NH₃)₆]³⁺ is:
✅ Answer
C) Octahedral
15. Which valency determines the geometry of a coordination compound?
✅ Answer
B) Secondary valency
16. Which species is the complex ion in [Co(NH₃)₆]Cl₃?
✅ Answer
C
17. Which of the following satisfies the primary valency of Co?
✅ Answer
B
18. Which of the following satisfies the secondary valency?
✅ Answer
B
19. The ionisable ions in [Co(NH₃)₆]Cl₃ are:
✅ Answer
C) Cl⁻
20. Correct representation of CoCl₃·6NH₃ is:
✅ Answer
B) [Co(NH₃)₆]Cl₃
🎯 Quick Revision
Werner Formula: [Co(NH₃)₆]Cl₃
Primary Valency: 3 → Oxidation State of Co
Primary Valency satisfied by: 3Cl⁻
Secondary Valency: 6 → Coordination Number
Secondary Valency satisfied by: 6NH₃
Coordination Entity: [Co(NH₃)₆]³⁺
Geometry: Octahedral
Ionisation: [Co(NH₃)₆]Cl₃ → [Co(NH₃)₆]³⁺ + 3Cl⁻
