Which is coloured because of d-d transition ?

Options

A. `KMnO_(4)`
B. `K_(2)CrO_(4)`
C. `CoCl_(3)`
D. All of these

Answer

To determine which compound is colored due to d-d transition, we need to analyze the nature of color in transition metal complexes and the specific compounds given in the options.

### Step-by-Step Solution:

1. **Understanding d-d Transition**:
- d-d transition occurs when an electron in a d orbital of a transition metal jumps from one d orbital to another d orbital of different energy levels. This transition absorbs specific wavelengths of light, leading to the appearance of color.

2. **Identifying the Compounds**:
- We need to evaluate each option provided to see if the color arises from d-d transitions or other mechanisms such as charge transfer.

3. **Analyzing Option 1: KMnO4 (Potassium Permanganate)**:
- KMnO4 is known to be colored, but its color arises from ligand-to-metal charge transfer, not from d-d transitions. The absorption of light occurs due to the interaction between the p orbitals of oxygen and the empty d orbitals of manganese.

4. **Analyzing Option 2: K2Cr2O7 (Potassium Dichromate)**:
- K2Cr2O7 is also a colored compound, but similarly, its color is due to charge transfer involving the oxygen atoms and chromium. Therefore, this option does not involve d-d transitions.

5. **Analyzing Option 3: CoCl3 (Cobalt(III) Chloride)**:
- CoCl3 contains cobalt in a +3 oxidation state, which has partially filled d orbitals. The presence of these d orbitals allows for d-d transitions, making this compound colored due to the electron jumping between d orbitals.

6. **Analyzing Option 4: All of These**:
- Since options 1 and 2 are not colored due to d-d transitions, this option cannot be correct.

7. **Conclusion**:
- The correct answer is **Option 3: CoCl3**, as it is colored due to d-d transitions.