AQA A-Level Chemistry Paper 3, June 2025: Question 17

✅ Correct Answer

C — optical only

💡 Key Knowledge

• Ligand nature: H₂NCH₂CH₂NH₂ (ethane-1,2-diamine, commonly abbreviated as "en") is a neutral bidentate ligand with two lone pairs on nitrogen atoms.
• Geometry & Coordination Number: 3 bidentate ligands form 6 dative covalent bonds to Cr³⁺ → octahedral geometry (coordination number = 6).
• Chirality / Optical Isomerism: An octahedral complex with 3 symmetrical bidentate rings, [M(en)₃]³⁺, adopts a "propeller-like" three-dimensional arrangement. It has no plane of symmetry, forming non-superimposable mirror images (enantiomers).
• Why no cis-trans isomerism? All three ligands are identical bidentate rings occupying all six positions symmetrically. There are no two distinct monodentate or mixed ligands that can be 90° (cis) or 180° (trans) relative to each other.

🧠 Exam Technique: The "Octahedral Rulebook"

Memorise these four standard AQA octahedral configurations to answer these 1-mark questions in under 15 seconds:

• [M(bidentate)₃]ⁿ⁺: Optical only (e.g. [Cr(en)₃]³⁺, [Fe(C₂O₄)₃]³⁻).
• [M(bidentate)₂(monodentate)₂]ⁿ⁺: Both cis-trans and optical (e.g. [Co(en)₂Cl₂]⁺; the cis isomer shows optical isomerism, whereas the trans isomer has a plane of symmetry and is optically inactive).
• [M(monodentate)₄(monodentate)₂]ⁿ⁺: cis-trans only (e.g. [Cr(H₂O)₄Cl₂]⁺).
• [M(monodentate)₆]ⁿ⁺: No stereoisomerism .

❌ Common Misconceptions & Traps

• Selecting D (cis-trans and optical): This is the most common wrong answer. Students confuse [M(en)₃]³⁺ with [M(en)₂Cl₂]⁺ , which does exhibit both.
• Selecting A (none): Students often forget that optical isomerism applies to coordination chemistry and look only for a tetrahedral carbon atom with four different groups (as in organic chemistry).
• Selecting B (cis-trans only): cis-trans requires a reference pair of identical groups that can be either 90° or 180° apart. In [Cr(en)₃]³⁺, all positions are connected identically.