Q.Suggest a reason as to why CO is poisonous.

Carbon monoxide has a lone pair on carbon that forms a strong coordinate bond with the Fe$^{2+}$ ion of haemoglobin, and this bond is reinforced by back-donation from filled Fe d-orbitals into CO's empty $\pi^*$ orbital. The resulting carboxyhaemoglobin is far more stable than oxyhaemoglobin and does not readily release, so affected haemoglobin can no longer carry O$_2$ to the tissues. …

1. Binding site. Haemoglobin carries oxygen by coordinating it at the Fe$^{2+}$ centre of its haem group.
2. Why CO binds so well. The carbon atom of CO has a lone pair that σ-donates into an empty Fe orbital, while filled Fe $d$-orbitals π-back-donate into CO's empty $\pi^*$ antibonding orbital — a synergic bonding pattern much like that in metal carbonyls. This makes the Fe–CO bond considerably stronger than the Fe–O$_2$ bond.
3. Consequence for stability. The resulting complex, carboxyhaemoglobin, is roughly 200 times more stable than oxyhaemoglobin, so once formed it does not dissociate to release the binding site back for O$_2$. …

Recall haemoglobin's oxygen transport role and compare the relative stability/bond strength of the Fe–CO vs Fe–O$_2$ coordinate bond (synergic σ-donation/π-back-donation, as in carbonyl complexes). Conclude that irreversible occupation of the Fe$^{2+ …

• Claiming CO is "directly toxic to tissues" rather than explaining it acts by blocking oxygen transport.
• Forgetting to mention the much greater stability of the Fe–CO bond compared to Fe–O$_2$ as the actual reason. …