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9701 · 28.4

Stereoisomerism in transition element complexes — practice questions

Practice and worked examples for 9701 Stereoisomerism in transition element complexes. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

The complex ion [Cr(H₂O)₄Cl₂]⁺ exists as two geometric isomers. Draw the 3D structures of these two isomers and label them as cis and trans.

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First, identify the geometry. With six ligands, it is octahedral. The isomers depend on the relative positions of the two Cl⁻ ligands.

Cis-isomer: The two Cl⁻ ligands are adjacent, at 90° to each other. We draw the Cr at the centre, four H₂O ligands, and two Cl ligands. Place one Cl on an axial position (top) and the other on an equatorial position (front-right, for example).

Trans-isomer: The two Cl⁻ ligands are opposite, at 180° to each other. Place one Cl on the top axial position and the other on the bottom axial position. The four H₂O ligands will then occupy the four equatorial positions.

(A diagram would show the octahedral shape with Cr at the centre. For the cis-isomer, Cl atoms would be at the 'top' and 'front-right' vertices. For the trans-isomer, Cl atoms would be at the 'top' and 'bottom' vertices. Both structures would be labelled clearly.)

Worked example 2

Draw the two optical isomers of the complex ion [Co(en)₃]³⁺. In your diagram, represent the bidentate ligand 1,2-diaminoethane (en) as a simple arc.

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  1. Start by drawing the central Co³⁺ ion and the six coordination bonds in an octahedral arrangement.
  2. For the first isomer, connect three pairs of adjacent coordination sites with arcs representing the 'en' ligands. For example, connect top to front-right, back-left to front-left, and back-right to bottom. This creates a 'right-handed propeller' twist.
  3. Draw a mirror plane (a vertical dashed line).
  4. Draw the mirror image of the first isomer. The central Co³⁺ is reflected. The arcs will now connect top to front-left, back-right to front-right, and back-left to bottom, creating a 'left-handed propeller' twist.
  5. Verify that the two structures are non-superimposable. No matter how you rotate the second structure, it will not perfectly overlap with the first.
  6. Label the diagrams as 'Optical Isomers' or 'Enantiomers'.
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