Explain optical isomerism in bidentate-ligand complexes
Recognise non-superimposable mirror-image complexes and distinguish optical isomerism from cis/trans isomerism.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 28. Use the first-row transition elements titanium to copper and the species explicitly named in the syllabus. Where redox potentials or stability constants are needed, use the values supplied in the question. Colour descriptions can depend on conditions and concentration. Written practice is self-checked
Before you begin
- Use electron configurations, oxidation numbers, ionic equations, equilibrium expressions and the AS redox and bonding models.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What relationship defines an enantiomer pair?
What you will learn
- Identify optical isomerism in [Ni(en)₃]²⁺ and [Ni(en)₂(H₂O)₂]²⁺
- Describe enantiomers as non-superimposable mirror images
- Use 3D arrangements to discuss polarity without equating chirality and polarity
Bidentate ligands can create handed arrangements
A complex can be optically isomeric when its three-dimensional structure and chelate arrangement have non-superimposable mirror images. The syllabus examples include [Ni(en)₃]²⁺ and [Ni(en)₂(H₂O)₂]²⁺.
The two forms are enantiomers. They have the same formula and connectivity but differ in spatial arrangement. A flat formula is not enough to distinguish them; trace how the bidentate ligands wrap around the metal in three dimensions.
Keep optical, geometrical and polarity descriptions distinct
Optical isomerism concerns mirror-image structures; cis/trans isomerism concerns relative ligand positions. A complex may have geometrical isomerism and some arrangements can also be optically active.
Polarity is determined by the vector sum of bond dipoles and the symmetry of the whole arrangement. Chirality alone does not guarantee that a molecule is polar; assess the actual complex structure when asked to deduce overall polarity.
Pause and explain
Which named complex is a syllabus example of optical isomerism?
Worked example
What structural feature makes the enantiomers of [Ni(en)₃]²⁺ optically isomeric?
Show the worked solution
- The bidentate ligands wrap around Ni in a three-dimensional arrangement.
- The mirror image cannot be superimposed by rotation alone.
- The two forms have the same composition.
Answer The three en ligands form a handed 3D arrangement. Its mirror image is non-superimposable, so the pair are optical isomers (enantiomers).
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State why [Ni(en)₃]²⁺ can form optical isomers.
Give me a hint
Consider the handed 3D arrangement made by three bidentate ligands.
Compare my reasoning
- Each en binds through two donor atoms.
- The ligands wrap around the central ion in 3D.
- The mirror image cannot be superimposed on the original.
Its bidentate ligands form a handed arrangement with a non-superimposable mirror image; the pair are optical isomers.
Look for these in your work
- Denticity understood
- 3D arrangement identified
- Mirror-image criterion stated
How does cis/trans isomerism differ from optical isomerism?
Give me a hint
Compare relative ligand positions with mirror-image superposition.
Compare my reasoning
- Cis/trans describes adjacent or opposite ligand positions.
- Optical isomerism describes non-superimposable mirror images.
- A complex may require a 3D drawing to assess either property.
Cis/trans is geometrical isomerism based on ligand positions; optical isomerism is a pair of non-superimposable mirror images.
Look for these in your work
- Position criterion stated
- Mirror criterion stated
- Three-dimensional structure recognised
A student observes that a complex is chiral and concludes it must be polar. Explain why this does not follow automatically.
Give me a hint
Chirality tests mirror-image superposition; polarity tests the net dipole vector.
Compare my reasoning
- A chiral arrangement can lack a mirror plane.
- Polarity depends on the vector sum of bond dipoles and symmetry.
- Assess the specific complex rather than infer polarity from chirality alone.
Chirality means the structure is non-superimposable on its mirror image. Polarity depends on the net dipole vector, so the actual ligand arrangement and symmetry must be analysed separately.
Look for these in your work
- Properties defined separately
- Dipole-vector criterion used
- Conclusion appropriately qualified
Common mistakes
- Calling mirror-image forms cis and trans without checking positions
- Treating a 2D formula as a full stereochemical drawing
- Claiming enantiomers have different formulae
- Assuming chirality alone proves polarity
What can you explain now?
What relationship defines an enantiomer pair?
Compare with the explanation
Non-superimposable mirror images
The pair have the same connectivity but opposite handed arrangements.
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Make it stick
What makes a complex optically isomeric, and why is that different from simply deciding whether it is polar?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.