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 of the given complexes is colored due to d-d transitions, we will analyze each option step by step.

### Step 1: Analyze K2MnO4
- **Oxidation State of Manganese**:
- Potassium (K) has a +1 charge, and there are 2 potassium ions, contributing +2.
- Oxygen (O) has a -2 charge, and there are 4 oxygen atoms, contributing -8.
- Let the oxidation state of manganese (Mn) be \( x \).

The equation is:
\[ x + 2 - 8 = 0 \implies x = +6 \]

- **Electron Configuration**:
- Manganese (Mn) has an atomic number of 25, so its electron configuration is \( \text{[Ar]} 3d^5 4s^2 \).
- In the +6 oxidation state, the configuration becomes \( \text{[Ar]} 3d^4 \).

- **Color Origin**:
- K2MnO4 is colored due to charge transfer from the ligand to the metal, not due to d-d transitions.

### Step 2: Analyze K2CrO4
- **Oxidation State of Chromium**:
- Similar to the previous calculation, with potassium contributing +2 and oxygen contributing -8.

The equation is:
\[ x + 2 - 8 = 0 \implies x = +6 \]

- **Electron Configuration**:
- Chromium (Cr) has an atomic number of 24, so its electron configuration is \( \text{[Ar]} 3d^5 4s^1 \).
- In the +6 oxidation state, the configuration becomes \( \text{[Ar]} 3d^0 \).

- **Color Origin**:
- K2CrO4 is yellow and is also colored due to charge transfer, not d-d transitions.

### Step 3: Analyze CoCl3
- **Oxidation State of Cobalt**:
- Chlorine (Cl) has a -1 charge, and there are 3 chlorine atoms, contributing -3.

The equation is:
\[ x - 3 = 0 \implies x = +3 \]

- **Electron Configuration**:
- Cobalt (Co) has an atomic number of 27, so its electron configuration is \( \text{[Ar]} 3d^7 4s^2 \).
- In the +3 oxidation state, the configuration becomes \( \text{[Ar]} 3d^6 \).

- **Color Origin**:
- In CoCl3, the 3d orbitals split into lower energy (t2g) and higher energy (eg) levels due to ligand field splitting.
- The electrons in the t2g orbitals can be excited to the eg orbitals, resulting in d-d transitions, which are responsible for the color of CoCl3.

### Step 4: Conclusion
- **Final Answer**: The complex that is colored due to d-d transitions is **CoCl3** (Option C).

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