How many distinct stereoisomers (including both geometrical and optical) are possible for the octahedral complex [M(AA)(BB)] where AA = ethylenediamine (en) and BB = oxalate (ox), both bidentate ligands that are identical within each type? Assume the complex is neutral and the metal is in the +2 oxidation state.
Answer
4 distinct stereoisomers (2 geometrical isomers that are achiral and 1 geometrical isomer that exists as a pair of enantiomers)
Step-by-step solution
- Identify that both AA and BB are bidentate, occupying four coordination sites each, so the complex has the type M(AA)(BB).
- Draw all possible relative arrangements of the two en ligands and the two ox ligands around the octahedron. Three geometrical arrangements are possible: (i) both en ligands trans to each other and both ox ligands trans (trans‑trans), (ii) one en trans and the other en cis to the ox ligands (cis‑trans), and (iii) both en ligands cis to each other and both ox ligands cis (cis‑cis).
- Examine the symmetry of each arrangement. The trans‑trans and cis‑trans forms possess a plane (or centre) of symmetry and are therefore achiral. The cis‑cis arrangement lacks any improper symmetry element and is chiral, giving a pair of non‑superimposable enantiomers (Δ and Λ).
- Count the distinct stereoisomers: trans‑trans (1), cis‑trans (1), and the two enantiomers of cis‑cis (2). Total = 4 distinct stereoisomers.
Common mistake: Forgetting to count the optical pair arising from the cis‑cis arrangement
Concept tested: isomerism with two different bidentate ligands
Published · How this solution was prepared and checked
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