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A2 · Practice questions · Stereoisomerism in complexes

Spot the isomers.

Six original Cambridge-style questions on stereoisomerism in transition metal complexes.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Cis, trans, chiral.

01
Recall
[3 marks]

Draw the two isomers of Pt(NH3)2Cl2, name the type of isomerism and identify cisplatin.

  • Square planar Pt with the two Cl adjacent (Cl–Pt–Cl 90°): cisplatin. ✓
  • Square planar Pt with the two Cl opposite (180°): the trans isomer. ✓
  • Geometrical (cis-trans) isomerism. ✓
02
Explain
[3 marks]

Describe how cisplatin acts as an anticancer drug.

  • In the cell a chloride ligand is replaced by water (ligand exchange), making the platinum reactive. ✓
  • Pt forms dative bonds with nitrogen atoms on two adjacent guanine bases in DNA. ✓
  • This distorts the DNA so it cannot replicate and the cancer cell cannot divide. ✓
03
Analysis
[2 marks]

Explain why a tetrahedral complex [ZnCl2(NH3)2] does not show cis-trans isomerism, but square planar [Pt(NH3)2Cl2] does.

  • In a tetrahedron every ligand position is adjacent to the other three (all angles 109.5°), so only one arrangement exists. ✓
  • In a square plane two identical ligands can be adjacent (90°) or opposite (180°), giving two isomers. ✓
04
Analysis
[3 marks]

Draw the stereoisomers of [Co(en)2Cl2]+ and state the total number.

  • Trans isomer: the two Cl opposite each other, both en ligands in one plane; has a plane of symmetry so not chiral. ✓
  • Cis isomer: the two Cl at 90°; it is chiral and exists as two non-superimposable mirror images. ✓
  • Total = 3 stereoisomers. ✓
05
Explain
[2 marks]

Explain why [Ni(en)3]2+ shows optical isomerism.

  • The three bidentate en ligands wrap around Ni2+ like a propeller, in a left- or right-handed sense. ✓
  • The two forms are non-superimposable mirror images (no plane of symmetry), so they are enantiomers. ✓
06
Analysis
[2 marks]

Octahedral [Co(NH3)4Cl2]+ exists as two isomers, one violet and one green. Name the type of isomerism and explain why there are only two.

  • Geometrical (cis-trans) isomerism. ✓
  • The two Cl ligands can only be adjacent (90°, cis) or opposite (180°, trans); all other arrangements are the same as one of these after rotation. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.