AQA A-Level Chemistry Paper 3, June 2018: Question 27

1 mark · Medium difficulty · Multiple Choice

Identify which transition metal complex exists as optical isomers from the given formulas.

Practise this question

Question

Question 27 asks 'Which complex exists as optical isomers?' with four options: A [Ag(NH3)2]+, B [Co(C2O4)3]4-, C [Cu(EDTA)]2-, and D [Cu(NH3)4(H2O)2]2+.
Question text

27 Which complex exists as optical isomers?

[1 mark]

A [Ag(NH ) ]+

B [Co(C O ) ]4–

24 3

C [Cu(EDTA)]2–

D [Cu(NH ) (H O) ]2+

34 2 2

Mark scheme

Show the mark scheme Mark scheme for question 27 shows that both B and C are accepted as correct answers.

27 B & C

How to answer it

Optical Isomerism in Transition Metal Complexes

📌 What this question tests

This question assesses your ability to recognise stereoisomerism in transition metal complexes, specifically distinguishing between:

  • Optical isomerism (chirality): Non-superimposable mirror images commonly found in octahedral complexes with multidentate ligands.
  • Geometric (cis-trans) isomerism: Different spatial arrangements of ligands around square planar or octahedral centres.
  • Ligand types: Monodentate, bidentate (oxalate, C₂O₄²⁻), and hexadentate (EDTA⁴⁻).

Question 27 Analysis

Identifying complexes that exist as non-superimposable mirror images

✅ Correct Answers: B or C [1 mark]

The exam board accepted either B or C:

  • B: [Co(C₂O₄)₃]⁴⁻ — An octahedral complex with three bidentate oxalate ligands arranged in a chiral "propeller" shape.
  • C: [Cu(EDTA)]²⁻ — A hexadentate ligand wrapped around an octahedral copper centre that also lacks an internal plane of symmetry, making its mirror image non-superimposable.

💡 Key Knowledge

  • Optical Isomerism: Occurs when a complex and its mirror image are non-superimposable (no plane of symmetry).
  • Standard A-Level Pattern: Octahedral complexes with three bidentate ligands (e.g. [M(bidentate)₃] like [Co(C₂O₄)₃]⁴⁻ or [Fe(H₂NCH₂CH₂NH₂)₃]³⁺) always form optical isomers.
  • Coordination Number & Shape:
    • [Ag(NH₃)₂]⁺: Coordination number 2, linear.
    • [Co(C₂O₄)₃]⁴⁻: Coordination number 6 (3 × 2), octahedral.
    • [Cu(EDTA)]²⁻: Coordination number 6 (1 × 6), octahedral.
    • [Cu(NH₃)₄(H₂O)₂]²⁺: Coordination number 6, octahedral.

📐 Systematic Breakdown of All Options

  1. A: [Ag(NH₃)₂]⁺
    Linear geometry with a central mirror plane perpendicular to the H₃N—Ag—NH₃ axis. Symmetrical and achiral; cannot show stereoisomerism.
  2. B: [Co(C₂O₄)₃]⁴⁻
    Three bidentate oxalate (C₂O₄²⁻) chelate rings bridge the coordination sites of the octahedral metal centre. They wind in either a left-handed or right-handed screw sense (like blades of a fan), yielding a pair of enantiomers (optical isomers).
  3. C: [Cu(EDTA)]²⁻
    EDTA⁴⁻ binds via 2 nitrogen and 4 oxygen atoms. The chelating cage wraps around the metal without an internal plane or centre of inversion, creating a chiral complex that exists as optical isomers.
  4. D: [Cu(NH₃)₄(H₂O)₂]²⁺
    An octahedral complex of formula [MA₄B₂]. This exhibits geometric (cis-trans) isomerism, but neither the cis nor trans isomer is chiral because both have internal planes of symmetry.

🧠 Exam Technique & Examiner Insight

  • The "B & C" Mark Scheme Story: B was the intended syllabus answer ([M(bidentate)₃] is the explicit textbook example for optical isomerism). However, [Cu(EDTA)]²⁻ is also scientifically chiral. AQA updated the mark scheme to credit both B and C so no student was penalised.
  • Shortcut Rule: If you see an octahedral metal with 3 bidentate ligands, immediately flag it as displaying optical isomerism!

❌ Common Errors

  • Confusing Optical with Geometric: Choosing D because it has isomers. Remember: [Cu(NH₃)₄(H₂O)₂]²⁺ forms cis/trans isomers, NOT optical isomers!
  • Missing Chelation Multiplicity: Forgetting that oxalate (C₂O₄²⁻) is bidentate, meaning 3 ligands satisfy a full coordination number of 6.
  • Assuming all octahedral complexes with different ligands are chiral: Monodentate ligands in [MA₄B₂] or [MA₃B₃] only give geometric isomers.
Examiner Summary: 1 mark was awarded for either B or C. Selecting D was the most frequent distractor among students who conflated geometric and optical isomerism.

Topics

Inorganic Chemistry · 3.2.5 Transition Metals

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.