Haemoglobin

Oxygen acts as a ligand by donating a lone pair of electrons to the $Fe^{2+}$ center to form a dative covalent bond.

This bond is relatively weak, which is a critical functional requirement; it must be strong enough to pick up oxygen in the lungs but weak enough to release it in tissues with low oxygen concentration.

When oxygen binds, the complex is called oxyhaemoglobin; when it is released, the site is often occupied by a water molecule or remains vacant until another gas molecule binds.

Carbon monoxide is a much better ligand than oxygen because it forms a significantly stronger dative bond with the $Fe^{2+}$ ion.

This process is a classic example of ligand exchange, where $CO$ replaces $O_2$ and refuses to leave, leading to carboxyhaemoglobin formation and potential suffocation at the cellular level.

Identify the Metal: Always specify that the central metal is Iron(II) ($Fe^{2+}$). Mentioning $Fe^{3+}$ is a common error that loses marks as it cannot bind oxygen effectively.

Coordination Numbers: Remember that while the haem group itself is tetradentate (4 bonds), the total coordination number of the iron in the body is 6 (octahedral).

Ligand Terminology: Use the term multidentate to describe the haem group and dative covalent bond (or coordinate bond) to describe the interaction between the ligands and the iron.

Color Changes: Note that the replacement of $O_2$ with $CO$ results in a color change to a darker, cherry-red, which is a diagnostic sign of carbon monoxide poisoning.

Oxidation State Confusion: Students often assume the iron is oxidized to $Fe^{3+}$ when oxygen binds. In reality, it remains $Fe^{2+}$; if it were oxidized, the binding would no longer be reversible.

Bond Type: Do not describe the iron-oxygen bond as ionic or simple covalent. It is a coordinate bond formed by electron pair donation from the oxygen molecule.

Ligand Count: Avoid saying haemoglobin has only one ligand. It has one large multidentate ligand (haem) and two monodentate ligands (globin and $O_2$).