Which of the following is formed in solution when `[Cu(NH_(3))_(4)]^(2+)` is treated with KCN till the colour of the complex is discharged?

Options

A. `Cu(CN)_(2)`
B. `[Cu(CN)_(4)]^(2-)`
C. `[Cu(CN)_(4)]^(3-)`
D. `[Cu(CN)_(6)]^(4-)`

Answer

To solve the problem of what is formed when \([Cu(NH_3)_4]^{2+}\) is treated with KCN until the color of the complex is discharged, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Initial Complex**: The initial complex is \([Cu(NH_3)_4]^{2+}\). This complex contains copper in the +2 oxidation state and is blue in color due to the presence of unpaired electrons.

2. **Understand the Reaction with KCN**: When \([Cu(NH_3)_4]^{2+}\) is treated with KCN, the cyanide ions (CN⁻) can replace the ammonia ligands in the complex. The reaction continues until the color of the complex is discharged, indicating a change in the oxidation state of copper.

3. **Determine the Oxidation State Change**: In the presence of excess KCN, copper is reduced from +2 to +1 oxidation state. The complex will change from \([Cu(NH_3)_4]^{2+}\) to \([Cu(CN)_4]^{3-}\). The copper in the +1 state has a completely filled d-orbital configuration (3d¹⁰), which is colorless.

4. **Formation of the Final Complex**: The final complex formed is \([Cu(CN)_4]^{3-}\). This complex is colorless, which aligns with the condition that the color of the complex is discharged.

5. **Select the Correct Option**: Among the given options, the complex formed is \([Cu(CN)_4]^{3-}\), which corresponds to option 3.

### Final Answer:
The complex formed when \([Cu(NH_3)_4]^{2+}\) is treated with KCN until the color is discharged is \([Cu(CN)_4]^{3-}\).

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