Stereoisomerism in transition element complexes
Cambridge A-Level Chemistry (9701) · Unit 28: Chemistry of transition elements · 7 flashcards
Stereoisomerism in transition element complexes is topic 28.4 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 28 — Chemistry of transition elements , alongside Colour of complexes and Stability constants, Kstab. In one line: [Pt(NH₃)₂Cl₂] exhibits geometrical (cis/trans) isomerism due to its square planar geometry. The cis isomer has both NH₃ ligands (or both Cl ligands) adjacent to each other, while the trans isomer has them opposite each other.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 7 flashcards — 2 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
What type of isomerism is shown by [Pt(NH₃)₂Cl₂]
[Pt(NH₃)₂Cl₂] exhibits geometrical (cis/trans) isomerism due to its square planar geometry. The cis isomer has both NH₃ ligands (or both Cl ligands) adjacent to each other, while the trans isomer has them opposite each other.
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Describe the kinds of stereoisomerism complexes can show, including those involving bidentate ligands: (a) cis/trans (geometrical) isomerism, e.g. square planar [Pt(NH₃)₂Cl₂], and octahedral [Co(NH₃)₄(H₂O)₂]²⁺ and [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺; (b) optical isomerism, e.g. [Ni(H₂NCH₂CH₂NH₂)₃]²⁺ and [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺.
- Deduce whether complexes like the cis/trans and optical isomer examples in the previous outcome are polar overall.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Stereoisomerism in transition element complexes
- › Carefully draw cis and trans isomers of octahedral complexes using wedges and dashes to show the different spatial arrangements.
What type of isomerism is shown by [Pt(NH₃)₂Cl₂]?
[Pt(NH₃)₂Cl₂] exhibits geometrical (cis/trans) isomerism due to its square planar geometry. The cis isomer has both NH₃ ligands (or both Cl ligands) adjacent to each other, while the trans isomer has them opposite each other.
Draw and label the cis and trans isomers of [Co(NH₃)₄(H₂O)₂]²⁺.
In the cis isomer, the two H₂O ligands are adjacent to each other. In the trans isomer, the two H₂O ligands are opposite each other.
Explain why [Ni(H₂NCH₂CH₂NH₂)₃]²⁺ exhibits optical isomerism.
[Ni(H₂NCH₂CH₂NH₂)₃]²⁺ exhibits optical isomerism because the complex is chiral. The three bidentate ligands coordinate to the central Ni²⁺ ion in a way that the molecule is non-superimposable on its mirror image.
Describe the difference in polarity between cis and trans isomers of [Pt(NH₃)₂Cl₂].
The cis isomer of [Pt(NH₃)₂Cl₂] is polar because the bond dipoles do not cancel out due to the adjacent arrangement of ligands. The trans isomer is non-polar because the bond dipoles cancel out due to the opposing arrangement of ligands.
What are the requirements for a complex to exhibit geometrical isomerism?
For geometrical isomerism (cis/trans), the complex must have two or more different ligands attached to the central metal ion and the ligands must be arranged in such a way that different spatial arrangements are possible. Typically seen in square planar and octahedral complexes.
Explain why some octahedral complexes containing only monodentate ligands can display geometric isomerism.
Octahedral complexes with monodentate ligands can display cis/trans isomerism if there are at least two of each type of ligand present. This allows for different spatial arrangements around the central metal ion.
Is [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺ chiral? Explain.
Only the cis isomer is chiral. In cis-[Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺ the two H₂O ligands are adjacent, so the complex has no plane of symmetry and is non-superimposable on its mirror image, giving two optical isomers. The trans isomer, with the H₂O ligands opposite each other, has a plane of symmetry and is not chiral.
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