Haem is an iron(II) complex with a multidentate ligand.

The idea

Haem is an octahedral iron(II) complex. The multidentate ligand is a porphyrin ring, which donates four lone pairs from four nitrogen atoms and so occupies four of the six co-ordination sites, all in one plane. The fifth site is filled by a nitrogen atom from the protein globin, which anchors the haem group in place. That leaves exactly one site free, and this sixth position holds a water molecule when no oxygen is present and an oxygen molecule when oxygen is available. Haemoglobin is the whole assembly, four haem groups held within one globin protein, so a single haemoglobin can carry up to four O₂ molecules. The iron remains Fe²⁺ throughout and is not oxidised when oxygen binds.

What earns the marks

• Identifying the oxidation state of the iron in haem as +2.
• Stating that the porphyrin ring is multidentate and donates four lone pairs from four nitrogen atoms.
• A co-ordination number of 6 and an octahedral shape.
• Naming globin as the fifth ligand and oxygen or water as the sixth.

Common mistakes and how to put them right

• Describing haem as an iron(III) complex, or saying the iron is oxidised when oxygen binds.

• Calling the porphyrin ring bidentate because it looks like one flat ligand.

• Confusing haem with haemoglobin, which also contains the globin protein.

• Giving the co-ordination number as 4 by counting only the nitrogen atoms of the ring.

Exam tips

• Account for all six co-ordination sites when describing haem: four ring nitrogens, one globin, one O₂ or H₂O.
• Keep the iron as Fe²⁺ in every statement you write about oxygen transport.
• Use the word multidentate or tetradentate about the ring rather than describing it as large.