AQA A-Level Chemistry Paper 3, June 2025: Question 32

1 mark · Easy difficulty · Multiple Choice

Identify the correct structure of transplatin from given square planar and tetrahedral representations.

Practise this question

Question

Multiple-choice question asking: 'Which structure is correct for transplatin?' Four molecular structures containing a central platinum atom bonded to two chlorine atoms and two ammonia ligands are shown. Structure A is square planar with adjacent chlorine ligands and adjacent ammonia ligands (cis). Structure B is square planar with chlorine ligands opposite each other and ammonia ligands opposite each other (trans). Structures C and D show tetrahedral-like geometries with wedge-and-dash bonds.
Question text

32 Which structure is correct for transplatin?

[1 mark]

A

B

C

D

Mark scheme

Show the mark scheme Mark scheme table indicating for question 32 the correct answer is B, awarded 1 mark (AO1), displaying the square planar trans isomer with chlorine and ammonia ligands diagonally opposite each other.

32 B 1 (AO1)

How to answer it

Transition Metals: Geometric Isomerism in Platinum(II) Complexes

What this question tests

This question evaluates your foundational recall and structural understanding of 4-coordinate platinum(II) complexes, specifically:

  • Recognising the characteristic square planar geometry of Pt(II) complexes.
  • Distinguishing between cis- and trans- stereoisomerism in square planar complexes.
  • Identifying the specific structure of transplatin compared to the anti-cancer drug cisplatin.
Question 32 • 1 Mark (AO1)

Identifying the Structure of Transplatin

Which structure is correct for transplatin?

✅ Correct Answer: B

Option B is the correct structure.

  • Geometry: Pt(II) in [Pt(NH₃)₂Cl₂] forms a square planar complex with 90° and 180° bond angles (ruling out C and D).
  • Stereoisomer: In transplatin, identical ligands are arranged directly opposite each other across the metal centre (180° apart). The two Cl⁻ ligands are diagonally opposite, and the two NH₃ ligands are diagonally opposite.

💡 Key Knowledge

  • Square Planar Platinum(II): Pt²⁺ has a d⁸ electron configuration and typically forms 4-coordinate complexes with square planar geometry.
  • Cisplatin vs Transplatin:
    • Cisplatin (Option A): Identical ligands are adjacent (90° apart). It binds specifically to adjacent guanine bases in DNA, acting as a potent anti-cancer chemotherapy drug.
    • Transplatin (Option B): Identical ligands are opposite (180° apart). It cannot form the same intra-strand DNA cross-links and is clinically inactive as an anti-cancer drug.
  • Tetrahedral complexes do not show cis/trans isomerism: All bond angles are equal (109.5°), and all positions are adjacent to one another.

🧠 Exam Technique: Two-Step Elimination

  1. Step 1: Check the geometry. Recall that Pt(II) complexes are square planar, not tetrahedral. Eliminate options C and D immediately.
  2. Step 2: Check the isomer prefix.
    • cis- = same side (adjacent).
    • trans- = across / opposite.
    Option A has both Cl ligands adjacent (cisplatin). Option B has Cl ligands diagonally opposite (transplatin). Choose B.

❌ Common Distractors & Errors

  • Selecting A (Cisplatin): A frequent misread error. Cisplatin is far more prominent in the syllabus due to its clinical role in cancer treatment, so students often rush and select cisplatin out of habit without noticing the "trans-" prefix.
  • Selecting C or D (Tetrahedral): Assuming that 4-coordinate complexes are always tetrahedral (like [CoCl₄]²⁻ or [CuCl₄]²⁻). Platinum(II) and nickel(II)/palladium(II) complexes often adopt square planar shapes.
  • Thinking tetrahedral complexes have geometric isomers: Mistaking a 3D wedge-and-dash tetrahedral representation for a trans-arrangement.
Mark Scheme Breakdown: [1 mark] for selecting B. (AO1 - simple recall of complex ion shapes and isomerism).

Topics

Inorganic Chemistry · Organic Chemistry · 3.2.5 Transition Metals · 3.3.13 Amino Acids, Proteins and DNA

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.