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On the basis of its interaction with haemoglobin, explain the toxicity of carbon monoxide.

I understand that ligand substitution is involved and carboxyhaemoglobin is formed meaning that the haem group in the haemoglobin molecule cannot form a ligand with the oxygen molecule, but i would like to know more in depth in terms of its chemistry, why this is the case.

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Carbon Monoxide and dioxygen both reversibly bind to the iron atom in hemoglobin.

The two molecules compete for the same binding site.

So if both $\ce{O2}$ and $\ce{CO}$ are present, some of each will bind, but for a given amount of $\ce{O2}$, less $\ce{O2}$ will bind if $\ce{CO}$ is present. The affinity of the Fe for $\ce{CO}$ is much greater than for $\ce{O2}$, so a lower concentration of $\ce{CO}$ results in a given saturation level of hemoglobin compare to $\ce{O2}$.

The same would be true for cyanide which is isoelectronic with carbon monoxide.

A more complete explanation needs to go into the cooperation between the four hemoglobin subunits. $\ce{CO}$ bound to two of the four subunits causes $\ce{O2}$ to being too tightly bound in the other subunits. That the oxygen can not be released from the hemoglobin, is another aspect of toxicity.

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