Which one of the following will give a white precipitate with `AgNO_3` in aqueous medium ?

Options

A. `[Co(NH_3)_5Cl](NO_2)_2`
B. `[Pt(NH_3)_2Cl_2]`
C. `[Pt(en)Cl_2]`
D. `[Pt(NH_3)_4]Cl_2`

Answer

To solve the question, we need to identify which of the given complexes will dissociate to release Cl⁻ ions in an aqueous medium, as these ions will react with AgNO₃ to form a white precipitate of silver chloride (AgCl).

### Step-by-Step Solution:

1. **Understanding the Reaction**:
- The formation of a white precipitate with AgNO₃ occurs when chloride ions (Cl⁻) are present in the solution. The reaction is:
\[ \text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl (s)} \]
- Therefore, we need to find a complex that can release Cl⁻ ions upon dissociation.

2. **Analyzing the Options**:
- **Option 1**: \( \text{[CO(NH}_3\text{)}_5\text{Cl(NO}_2\text{)]}^{2+} \)
- Here, Cl is part of the coordination sphere. When this complex dissociates, it does not release Cl⁻ ions. Therefore, it will not form a white precipitate.

- **Option 2**: \( \text{[Pt(en)}_2\text{Cl}_2] \)
- In this case, Cl is also inside the coordination sphere and will not dissociate to release Cl⁻ ions. Hence, it will not form a white precipitate.

- **Option 3**: \( \text{[Pt(NH}_3\text{)}_2\text{Cl}_2] \)
- Similar to the previous options, Cl is part of the coordination sphere and will not dissociate to release Cl⁻ ions. Thus, no white precipitate will form.

- **Option 4**: \( \text{[Pt(NH}_3\text{)}_4\text{Cl}_2] \)
- In this complex, Cl is outside the coordination sphere. Upon dissociation, it can release Cl⁻ ions:
\[ \text{[Pt(NH}_3\text{)}_4\text{Cl}_2] \rightarrow \text{[Pt(NH}_3\text{)}_4]^{2+} + 2\text{Cl}^- \]
- The released Cl⁻ ions will react with AgNO₃ to form AgCl, which is a white precipitate.

3. **Conclusion**:
- The only complex that can dissociate to release Cl⁻ ions and thus form a white precipitate with AgNO₃ is option 4: \( \text{[Pt(NH}_3\text{)}_4\text{Cl}_2] \).

### Final Answer:
The correct option is **Option 4: \( \text{[Pt(NH}_3\text{)}_4\text{Cl}_2] \)**.