Haemoglobin and Ligand Substitution Revision Notes | A Level AQA Chemistry

1. Why Haemoglobin Matters for Transition Metal Chemistry

Every breath you take relies on a transition metal complex doing its job with extraordinary precision. Haemoglobin — the protein that carries oxygen from your lungs to your tissues — works because of the chemistry of iron(II) and co-ordinate bonding. This is not just biology; it is a direct application of the transition metal chemistry you have been studying.

The same principles that govern ligand substitution in a test tube govern what happens when carbon monoxide enters your bloodstream. Understanding this at the molecular level is exactly what AQA expects you to do — and it is one of the most satisfying connections in the entire A-level specification.

2. The Structure of Haem: An Iron(II) Complex

At the heart of haemoglobin sits a structure called haem. Picture a flat, ring-shaped organic molecule — the porphyrin ring — with an iron(II) ion sitting at its centre. The porphyrin ring acts as a multidentate ligand, forming four co-ordinate bonds to the Fe(II) ion through four nitrogen donor atoms arranged in a square around the metal.

The porphyrin ring occupies four of the six co-ordination sites around the octahedral Fe(II) centre. A fifth site is occupied by a nitrogen atom from the protein chain (a histidine residue). This leaves the sixth co-ordination site free — and that is the site where oxygen binds. The entire architecture of haem is built to present that one vacant site to incoming ligands.

The fact that the porphyrin ring is multidentate is chemically significant. Multiple co-ordinate bonds from a single ligand create a very stable complex — this is the chelate effect. The Fe(II) centre is held firmly in place within the ring, which means the protein can position that sixth co-ordination site precisely where it needs to be for oxygen capture and release.

3. Oxygen Binding: Co-ordinate Bonding at Fe(II)

When oxygen reaches the haem group, it forms a co-ordinate bond to the Fe(II) ion at the sixth co-ordination site. O2 acts as a ligand, forming a co-ordinate bond to Fe(II) by donating electron density into an empty d-orbital on Fe(II), creating a dative covalent bond. This is the same type of bonding you have seen in other transition metal complexes — the ligand acts as an electron-pair donor and the metal ion acts as an electron-pair acceptor.

Crucially, this bond is reversible. In the lungs, where the partial pressure of oxygen is high, oxygen binds readily to Fe(II). In the tissues, where oxygen concentration is lower, the bond breaks and oxygen is released. The Fe(II) oxidation state is maintained throughout — haemoglobin does not oxidise to Fe(III) during normal oxygen transport, which would prevent oxygen release.

The reversibility of oxygen binding is what makes haemoglobin such an effective transport molecule. The co-ordinate bond is strong enough to hold oxygen during transport through the bloodstream, yet weak enough to break when oxygen is needed by respiring cells. This balance is a direct consequence of the electronic properties of Fe(II) and the geometry of the haem complex.

4. Carbon Monoxide Toxicity: Ligand Substitution Gone Wrong

Now that you understand how oxygen binds, the mechanism of carbon monoxide poisoning becomes clear — and it is a textbook example of ligand substitution. Carbon monoxide ($\text{CO}$) is also capable of donating a lone pair to the Fe(II) centre in haem. It competes directly with oxygen for that sixth co-ordination site.

The critical difference is that carbon monoxide binds to Fe(II) approximately 200–250 times more strongly than oxygen does. Once CO occupies the sixth co-ordination site, it is not readily displaced. The ligand substitution is effectively irreversible under physiological conditions, meaning that haemoglobin molecules bound to CO can no longer carry oxygen.

Worked example: Explaining CO Toxicity Using Ligand Substitution Principles

Explain, in terms of co-ordinate bonding and ligand substitution, why carbon monoxide is toxic. [4 marks]

1. In haemoglobin, O2 forms a co-ordinate bond to Fe(II) at the sixth co-ordination site of the haem group. — [B1] Establish the normal situation first — this gives the examiner context and earns a mark for identifying the co-ordinate bond to Fe(II).
2. CO also donates a lone pair to Fe(II) at the same co-ordination site, forming a co-ordinate bond and displacing O2 in a ligand substitution reaction. — [B1] Name the process as ligand substitution and state that CO donates a lone pair to Fe(II). Both details are needed for the mark.
3. CO binds to Fe(II) much more strongly than O2 does, so the substitution is not readily reversed under physiological conditions. — [B1] The relative binding strength is the key chemical reason for irreversibility. Examiners expect this comparison explicitly.
4. Haemoglobin molecules with CO bound at the haem site cannot bind or transport oxygen, reducing the oxygen-carrying capacity of the blood and causing toxicity. — [B1] Link the chemistry to the biological consequence. A complete answer must connect the molecular event to the physiological outcome.

Answer: CO donates a lone pair to Fe(II) in haemoglobin, forming a co-ordinate bond at the same site as O2 in a ligand substitution reaction. Because CO binds much more strongly than O2, this substitution is not readily reversed, so haemoglobin can no longer transport oxygen, causing toxicity.

Notice how the explanation above follows a clear chain: bonding type → process name → relative strength → consequence. That four-step logic is exactly what AQA mark schemes reward. A student who writes only "CO stops oxygen being carried" will score one mark at most; a student who traces the co-ordinate bonding and ligand substitution through to the physiological outcome will score full marks.

5. Comparing O2 and CO as Ligands: A Deeper Analysis

Both oxygen and carbon monoxide are ligands that bind to Fe(II) through lone pair donation, forming co-ordinate bonds. The difference lies in the strength of the interaction: CO forms a stronger co-ordinate bond to Fe(II) than O2 does. This means the Fe–CO bond requires considerably more energy to break than the Fe–O2 bond, which is why CO binding is effectively irreversible under physiological conditions. [Note: the detailed electronic reasons for CO's greater binding strength, including σ-donation and π back-bonding, are beyond the AQA A-level specification and are not required in exam answers.]

One detail worth noting: both O2 and CO leave the iron in the +2 oxidation state. This is different from what happens when haemoglobin is oxidised to methaemoglobin (Fe(III)), which is a separate pathological condition. For AQA purposes, the key distinction is the binding strength of CO versus O2, not any change in oxidation state.

6. Exam Technique: Scoring Full Marks on Haemoglobin Questions

AQA questions on this topic tend to use the command words explain, describe, or compare. Each demands a different response structure. An explain question requires you to give a reason using chemical principles — not just state what happens. A compare question requires you to address both species (O2 and CO) in the same sentence or adjacent sentences, making the contrast explicit.

Three phrases consistently appear in mark schemes for this topic: "co-ordinate bond" (or "dative covalent bond"), "lone pair donated to Fe(II)", and "ligand substitution". If any of these three phrases is absent from your answer, you are leaving marks on the table. Practise writing them until they appear automatically.

The model answer above earns all six marks because it addresses every required element in a logical sequence: structure of haem → normal O2 binding → CO binding mechanism → comparison of bond strengths → consequence. Notice that each sentence advances the argument rather than repeating the same point in different words. That economy of language is a mark-winning habit.

7. Bringing It Together: Spec Points in One View

Every AQA spec requirement for this topic connects through a single chemical thread: co-ordinate bonding at Fe(II). Haem provides the structural context (iron(II) complex with a multidentate ligand). Oxygen transport demonstrates the function of that co-ordinate bond. Carbon monoxide toxicity shows what happens when a stronger ligand undergoes substitution at the same site. These are not three separate facts — they are one story told at increasing depth.

• Haem: Fe(II) complex with porphyrin ring as a multidentate ligand (four N donor atoms, four co-ordinate bonds).
• Oxygen transport: O2 donates a lone pair to Fe(II) at the sixth co-ordination site, forming a reversible co-ordinate bond.
• CO toxicity: CO undergoes ligand substitution at the same Fe(II) site; the Fe–CO bond is much stronger than Fe–O2, making the substitution effectively irreversible and preventing oxygen transport.

Mastering this topic means being able to move fluently between the molecular level (lone pair donation, co-ordinate bond, ligand substitution) and the physiological level (oxygen transport, toxicity). AQA questions will often give you a novel context — a different metal complex or a different competing ligand — and ask you to apply the same principles. The underlying logic is always the same: identify the co-ordination site, identify the competing ligands, compare their binding strengths, and state the consequence.