The colour of a complex compound is due to .

Options

A. Promotion of 3d-electrons of the central atom//ion to 4p-orbitals.
B. Promation of 3d-electrons of the central atom//ion to 4s-orbitals
C. Promation of 3d-electrons of the central atom//ion within d-orbitals
D. Promation of 4s-electrons of the central atom//ion to 4p-orbitals

Explanation

**Step-by-Step Solution:**

1. **Understanding Color in Complex Compounds:**
- The color of a complex compound arises from the interaction of light with the electrons in the metal ion's d-orbitals.

2. **Key Phenomena Contributing to Color:**
- The two main phenomena that contribute to the color of coordination compounds are:
- **d-d Transition:** This involves the excitation of electrons within the d-orbitals of the metal ion.
- **Charge Transfer:** This involves the transfer of an electron from a ligand to the metal ion or vice versa, but is less common in determining the color of most complexes.

3. **D-D Transition Explained:**
- In transition metal complexes, the d-orbitals split into different energy levels due to the presence of ligands. When light hits the complex, certain wavelengths are absorbed to promote an electron from a lower energy d-orbital to a higher energy d-orbital.
- This transition is known as d-d transition and is responsible for the visible color of the complex.

4. **Conditions for Color:**
- D-d transitions can only occur if there are unpaired electrons in the d-orbitals. Therefore, complexes with d^0 (no d electrons) or d^10 (fully filled d orbitals) configurations are typically colorless, as there are no available electrons to excite.

5. **Conclusion:**
- The color of a complex compound is primarily due to the promotion of 3d electrons within the d-orbitals of the central metal ion, which leads to d-d transitions.

**Final Answer:**
The color of a complex compound is due to the promotion of 3D electrons of the central atom ion within the d-orbitals.

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