OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2019: Question 13

✅ Correct Answer

B (3)

The complex ion Ni(H₂NCH₂CH₂NH₂)₂Cl₂ forms a total of 3 stereoisomers: one trans-isomer (achiral) and a pair of cis-enantiomers (optical isomers).

💡 Key Knowledge

• H₂NCH₂CH₂NH₂ is 1,2-diaminoethane (often abbreviated as en ), a bidentate neutral ligand.
• Each bidentate ligand forms two coordinate bonds, meaning two en ligands account for 4 coordinate bonds, and two Cl⁻ ligands account for the remaining 2, giving an octahedral coordination number of 6.
• Octahedral complexes of type [M(bidentate)₂X₂] display both geometrical (cis/trans) and optical isomerism.

🧠 Exam Technique

Do not rush complex ion stereochemistry questions. Mentally construct (or quickly sketch on your rough paper) the 3D octahedral layout:

• Trans-isomer: The two Cl atoms are opposite each other (180° apart). This isomer has a plane of symmetry, so it is achiral (does not show optical activity).
• Cis-isomer: The two Cl atoms are adjacent (90° apart). Because of this unsymmetrical arrangement, the cis-form lacks a plane of symmetry and exists as a pair of non-superimposable mirror images (enantiomers).
• Total stereoisomers = 1 (trans) + 2 (cis enantiomers) = 3.

❌ Common Errors

• Forgetting optical isomers: Many students correctly identify the cis and trans structures (giving an answer of 2), completely missing that the cis-isomer is chiral and exists as two optical enantiomers.
• Miscounting coordination number: Assuming 1,2-diaminoethane is monodentate, leading to confusion over the overall geometry. Always check ligand denticity!