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5.6.3Physical Chemistry and Transition Elements

Isomerism in Transition Element Complexes

5 min read 4 parts Lesson 36 of 40

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Section 1 of 4Types of Isomerism in Complexes

5.6.3 Isomerism in Transition Element Complexes

Complexes with the same formula can differ in the arrangement of ligands in space.

Stereoisomers
  • Isomers with the same formula and the same bonds but a different arrangement of atoms in space.
Cis–trans isomerism
  • Occurs when two identical ligands are on the same side (cis) or on opposite sides (trans) of the central ion.
Square planar example
  • [Pt(NH₃)₂Cl₂] exists as cis, with the two Cl⁻ adjacent at 90°, and trans, with them opposite at 180°.
Octahedral example
  • [Co(NH₃)₄Cl₂]⁺ has a cis form and a trans form, depending on the positions of the two chloride ligands.

Section 2 of 4Optical Isomerism

Some octahedral complexes with bidentate ligands have non-superimposable mirror images.

1
A complex such as [Ni(NH₂CH₂CH₂NH₂)₃]²⁺ contains three bidentate ligands arranged around the metal like the blades of a propeller.
2
The structure and its mirror image cannot be placed exactly on top of each other, so the complex is chiral.
3
The two forms are enantiomers, and they rotate the plane of plane-polarised light in opposite directions.
4
Cis forms of octahedral complexes with two bidentate ligands, such as cis-[Co(en)₂Cl₂]⁺, are also optical isomers, whereas the trans form is not.
5
To draw them, show the three ligand rings in a three-dimensional arrangement with the mirror plane between them.

Section 3 of 4Cisplatin in Medicine

Cisplatin in Medicine
Structure
  • Cisplatin is cis-[Pt(NH₃)₂Cl₂], a neutral square planar complex of platinum(II).
Action
  • The chloride ligands are substituted by nitrogen atoms on guanine bases in DNA, linking two bases in the same strand.
Result
  • The DNA can no longer be copied properly, so rapidly dividing cancer cells cannot replicate and die.
Side effects
  • Healthy dividing cells are also affected, causing nausea and hair loss, and the trans isomer is not effective.

Section 4 of 4Exam Checklist & Tips

5.6.3 – Key Points for the Exam

  • Stereoisomers Compounds with the same structural formula but different arrangements in space.
  • Cis isomer An isomer with identical ligands adjacent to each other.
  • Trans isomer An isomer with identical ligands opposite each other.
  • Enantiomers Non-superimposable mirror image isomers.
  • Cisplatin A platinum anti-cancer drug, cis-[Pt(NH₃)₂Cl₂].

Common Mistakes

  • Drawing cis–trans isomers in a tetrahedral complex, which cannot show this type of isomerism.
  • Describing optical isomers as having different formulae; they have identical formulae and bonding.
  • Saying that trans-platin has the same medical use as cisplatin.
  • Forgetting that optical isomers need the complex to be non-superimposable on its mirror image.
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