A level Chemistry · Edexcel 9CH0 · Topic 15B
Complex ions and ligands
Key points
- A ligand is a species with a lone pair that forms a dative bond to a metal ion. Coordination number = number of dative bonds.
- Monodentate ligands: H₂O, NH₃, Cl⁻, OH⁻. Bidentate: ethane-1,2-diamine, ethanedioate. Multidentate: EDTA⁴⁻ (hexadentate).
- Shapes: octahedral (six small ligands, 90°), tetrahedral (four large ligands like Cl⁻, 109.5°), square planar (e.g. cisplatin, 90°), linear (e.g. [Ag(NH₃)₂]⁺).
- Stereoisomerism: cis/trans in octahedral and square planar complexes; optical isomers with three bidentate ligands.
- Cisplatin binds to DNA and stops cancer cells dividing, but has side effects.
- Haem contains Fe²⁺; CO binds more strongly than O₂, which makes CO toxic.
- Replacing monodentate ligands with multidentate ones is favourable because the number of particles increases, so entropy rises (the chelate effect).
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A complex ion is a central metal ion surrounded by ligands joined to it by dative bonds. This sub-topic covers how ligands bond, the shapes and isomers that result, two complexes that matter in medicine and biology, and why multidentate…
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