How many distinct stereoisomers (including optical isomers) are possible for the octahedral complex [M(NH₃)₄Cl₂] where M = Fe³⁺ and the two Cl ligands are identical while the four NH₃ ligands are identical?
Answer
3
The correct answer is 3 — Add the number of isomers: 1 (trans) + 2 (cis enantiomers) = 3 stereoisomers.
Step-by-step solution
- Recognise that the complex has an octahedral geometry with two identical ligands (Cl) and four identical ligands (NH₃).
- Count the possible geometrical arrangements of the two Cl ligands: cis (adjacent) and trans (opposite).
- Analyse the cis arrangement for chirality: the lack of a mirror plane or center of symmetry makes it chiral, giving two enantiomers (Δ and Λ).
- Analyse the trans arrangement: it possesses a plane of symmetry and is therefore achiral, giving only one isomer.
- Add the number of isomers: 1 (trans) + 2 (cis enantiomers) = 3 stereoisomers.
Common mistake: Assuming the cis form is achiral and missing the optical isomers.
Concept tested: Stereochemistry of coordination complexes
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