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
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- describe the types of stereoisomerism shown by complexes, including those associated with bidentate ligands: (a) geometrical (cis/trans) isomerism, e.g. square planar such as [Pt(NH₃)₂Cl ₂] and octahedral such as [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 the overall polarity of complexes such as those described in 28.4.1(a) and 28.4.1(b)
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
- › Explain color as: d-orbitals split, light is absorbed for electron promotion/excitation, and the complementary color is seen.
- › Remember that electrons are removed from the 4s orbital before the 3d orbital when forming transition metal cations.
- › Check the group number to determine maximum oxidation states; for vanadium (Group 5), it is +5.
- › Define a ligand as a species that uses a lone pair of electrons to form a dative covalent bond to a metal ion.
- › Use wedges and dashed lines clearly to show three-dimensional tetrahedral structures with an angle of 109.5 degrees.
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.
Yes, [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺ is chiral and can exist as optical isomers. The two bidentate ligands and two water ligands create a non-superimposable mirror image.
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