Question

The red colour of oxyhaemoglobin is mainly due to

• A. d-d transition
• B. metal to ligand charge transfer transition
• C. ligand to metal charge transfer transition
• D. intraligand $\pi -{\pi }^{\ast }$ transition

Answer 1

Answer: (A)

When oxygen is bound to $F{e}^{2+}$ in heme, all the d-electrons of the Fe atom are forced into the three lower energy $t_{2g}$ orbitals and it produces low spin oxyhaemoglobin.

In oxyhaemoglobin, $F{e}^{2+}=\left[Ar\right]3d^6$



The sharp high energy transition between $t_{2g}$ and $e_g$ levels of the d-electrons is responsible for the bright red colour of oxyhaemoglobin.