Ammonia acts as a very good ligand but ammonium ion does not form complexes because

Options

A. `NH_(3)` is a gas while `NH_(4)^(+)` is in liquid form
B. `NH_(3)` undergoes `sp^(3)` hybridisation while `NH_(4)^(+)` undergoes `sp^3` d hybridisation
C. `NH_(4)^(+)` ion does not have any lone pair of electrons
D. `NH_(4)^+` ion has one unpaired electron while `NH_(3)` has two unpaired electrons .

Answer

To solve the question of why ammonia acts as a very good ligand while the ammonium ion does not form complexes, we can break down the reasoning step by step.

### Step-by-Step Solution:

1. **Understanding Ligands**:
- A ligand is a species that can donate a pair of electrons to a metal ion to form a coordination complex.
- For a ligand to effectively coordinate with a metal ion, it must have available lone pairs of electrons.

2. **Analyzing Ammonia (NH₃)**:
- Ammonia is a neutral molecule with a nitrogen atom that has one lone pair of electrons.
- This lone pair allows ammonia to act as a Lewis base, donating the lone pair to a metal ion and forming a coordinate bond.

3. **Analyzing Ammonium Ion (NH₄⁺)**:
- The ammonium ion is a positively charged species formed when ammonia accepts a proton (H⁺).
- In the ammonium ion, the nitrogen atom has no lone pairs of electrons available for donation because it has already formed four bonds (three with hydrogen atoms and one with the additional proton).

4. **Conclusion**:
- Since the ammonium ion lacks lone pairs of electrons, it cannot act as a ligand and therefore does not form coordination complexes.
- In contrast, ammonia, with its lone pair, can effectively coordinate with metal ions.

### Final Answer:
Ammonia acts as a very good ligand because it has a lone pair of electrons available for donation, while the ammonium ion does not form complexes because it lacks lone pairs of electrons.