The red colour of oxyhaemoglobin is mainly due to

Options

A. d - d transition
B. metal to ligand charge transfer transition
C. ligand to metal charge transfer transition
D. intraligand `pi-pi^(**)` transition

Answer

To solve the question regarding the red color of oxyhaemoglobin, we will analyze the different types of electronic transitions that can occur in the molecule.

### Step-by-Step Solution:

1. **Understanding Hemoglobin**:
Hemoglobin is a protein in red blood cells that carries oxygen. It contains iron ions (Fe²⁺) that bind to oxygen. The color of hemoglobin changes based on its oxidation state and the ligands attached to it.

**Hint**: Remember that the color of a compound is often related to the electronic transitions of its metal ions.

2. **Identifying Types of Electronic Transitions**:
- **dd Transition**: This involves the transition of electrons between different d-orbitals within the same metal ion. This transition is common in transition metal complexes but does not primarily account for the color of oxyhaemoglobin.

**Hint**: Consider how d-orbitals are split in the presence of ligands and how this affects color.

- **Metal to Ligand Charge Transfer (MLCT)**: This transition involves the transfer of an electron from a metal's molecular orbital to a ligand's molecular orbital. While this can contribute to color, it is not the primary reason for the red color in oxyhaemoglobin.

**Hint**: Think about the nature of the ligands and their ability to accept electrons from the metal.

- **Ligand to Metal Charge Transfer (LMCT)**: This is the opposite of MLCT, where an electron is transferred from a ligand to a metal. This type of transition is also not responsible for the red color of oxyhaemoglobin.

**Hint**: LMCT is less common in biological systems compared to MLCT.

- **Intra-Ligand π-π* Transition**: This transition involves the movement of electrons from a bonding π orbital to an anti-bonding π* orbital within the same ligand. This transition is significant in the context of hemoglobin.

**Hint**: Consider how the structure of hemoglobin and its ligands can facilitate this type of transition.

3. **Conclusion**:
The red color of oxyhaemoglobin is mainly due to the intra-ligand π-π* transition. This transition occurs when the electrons in the π bonding orbitals of the heme group (the iron-containing part of hemoglobin) are excited to the π* anti-bonding orbitals, resulting in the characteristic red color.

**Final Answer**: The red color of oxyhaemoglobin is mainly due to **intra-ligand π-π* transition**.