Isomerism in Transition Element Complexes (OCR A Level Chemistry)

• Transition element complexes can exhibit stereoisomerism

 Geometrical (cis-trans) isomerism

• Even though transition element complexes do not have a double bond, they can still have geometrical isomers
• Square planar and octahedral complexes with two pairs of different ligands exhibit cis-trans isomerism
• An example of a square planar complex with two pairs of ligands is the anti-cancer drug cis-platin
  • Whereas cis-platin has beneficial medical effects by binding to DNA in cancer cells, trans-platin cannot be used in cancer treatment

Cis-platin is an example of a square planar transition element complex that exhibits geometrical isomerism

• As long as a complex ion has two ligands attached to it that are different to the rest, then the complex can display geometric isomerism
• Examples of octahedral complexes that exhibit geometrical isomerism are the [Co(NH₃)₄(H₂O)₂]²⁺ and [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺ complexes
  • [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺ can also be written as [Ni(en)₂(H₂O)₂]²⁺

• Like in the square planar complexes, if the two ‘different’ ligands are next to each other then that is the ‘cis’ isomer, and if the two ‘different’ ligands are opposite each other then this is the ‘trans’ isomer
  • In [Co(NH₃)₄(H₂O)₂]²⁺, the two water ligands are next door to each other in the cis isomer and are opposite each other in the trans isomer

Octahedral transition metal complexes exhibiting geometrical isomerism

Optical isomerism

• Octahedral complexes with bidentate ligands also have optical isomers
• This means that the two forms are non-superimposable mirror images of each other
  • They have no plane of symmetry, and one image cannot be placed directly on top of the other

• The optical isomers only differ in their ability to rotate the plane of polarised light in opposite directions
• Examples of octahedral complexes that have optical isomers are the [Ni(H₂NCH₂CH₂NH₂)₃]²⁺and [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺ complexes
  • The ligand H₂NCH₂CH₂NH₂ can also be written as ‘en’ instead

Octahedral transition metal complexes exhibiting optical isomerism

• In the 1960's, the drug cis-platin was discovered
• Cis-platin has been extremely effective in treating a number of different types cancer such as testicular, ovarian, cervical, breast, lung and brain cancer
  • Cancer cells grow and replicate much faster than normal cells

• The cis-platin works by binding to the nitrogen atoms on the bases in DNA
• The cis-platin passes through the cell membrane and undergoes ligand exchange where the chlorines are replaced by water molecules
• The nitrogen is a better ligand than water and forms dative covalent bonds with the cis-platin
• The cis-platin distorts the shape of the DNA and prevents the DNA from replicating

Adverse Effects

• Cis-platin binds to healthy cells as well as cancerous cells, but affects cancer cells more as they are replicating faster
  • Unfortunately, this means that other healthy cells which replicate quickly, such as hair follicles, are also affected by cis-platin
  • This is why hair loss is a side-effect of people undergoing cancer treatment
• Despite this drawback, cis-platin is a highly effective drug and society needs to find a balance between the adverse effects of drugs and their therapeutic value
• New therapeutic pathways are constantly under development that aim to deliver drugs that target cancer cells while leaving healthy cells untouched