Edexcel IAL Chemistry 17.17 haemoglobin and ligand exchange

Focus on the Fe2+ complex: oxygen binds reversibly, but carbon monoxide replaces oxygen as a ligand and forms stronger dative bonding in carboxyhaemoglobin.

Exam points

Identify carbon monoxide binding to oxyhaemoglobin as ligand exchange.
Explain toxicity through CO replacing oxygen and binding more strongly to Fe2+.
State that oxygen binds reversibly to the iron(II) ion in haemoglobin.

[Maximum number: 2]

This question is about compounds and complex ions of iron.

Haemoglobin is an iron(II) complex which carries oxygen around the body. Part of the structure of oxyhaemoglobin is shown.

Figure for Question 17.17—Haemoglobin is an iron(II) complex containing a polydentate ligand and that ligand exchange occurs when an oxygen molecule question 1 — Edexcel A-Level Chemistry A2

The four nitrogen atoms are part of a multidentate ligand in the haem group.
Explain, in terms of the iron(II) complex, why carbon monoxide is toxic.

An explanation that makes reference to the following points:

- carbon monoxide replaces / takes the place of the oxygen molecule / ligand

(1)

- (and it may be toxic) because it binds strongly to the

(1) $\mathrm{Fe}^{2+}$ ion

Accept ligand substitution / exchange reaction between oxygen and carbon monoxide COMMENT

The question refers to oxygen being carried around and so there needs to be explicit reference and not just implied that to it being replaced/substituted or its place being taken

(2)

Figure for Answer to Question 17.17—Haemoglobin is an iron(II) complex containing a polydentate ligand and that ligand exchange occurs when an oxygen molecule question 1 — Edexcel A-Level Chemistry A2

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17.17—Haemoglobin is an iron(II) complex containing a polydentate ligand and that ligand exchange occurs when an oxygen molecule question 1