Chemqueries: Stereoisomerism

Tuesday, 20 November 2018


  • The isomer in which same type and number of ligands coordinated to the metal atom or cation but with different spatial(spatial arrangement means an arrangement in space)arrangements are called stereoisomers.
  • Stereoisomerism is classified into two types i.e.
    1. Geometrical isomerism
    2. Optical isomerism

Geometrical isomerism:

  • Stereoisomers in which relative position or orientation of the ligand is different i.e. donor atom around the central metal cation is different are called geometrical isomers and the phenomenon is called geometrical isomerism.
  • Geometrical isomers cannot be interconverted without breaking the metal-ligand(M-L) bond.
  • Geometrical isomerism is shown by that compound which can be converted into its cis and transform.
  • The isomer in which two particular ligands occupy the adjacent position of each other is called cis- isomer and the isomer in which two adjacent ligands occupy an opposite position to each other is called trans -isomer.
  • Cis and trans isomer are different compounds with different properties like melting point, dipole moment, solubility, colours, and chemical properties.
  • Geometrical isomerism is most common in complexes having coordination number 4 and 6 but the complexes having coordination number 2 and 3 do not exhibit geometrical isomerism.
  • cis/trans-2-butene

  • But in the coordination compound, this type of isomerism is found mainly in the heteroleptic complex because here multiple geometrical arrangements of ligand around the central metal atom are possible.
  • Square planar complexes are coordination compounds with coordination number 4 having [MX2L2] type formula, where X and L are unidentate ligands. The two ligands X could either be adjacent to each other in a cis isomer or opposite to each other to form a trans isomer.
    Square planar complexes with MABXL type formula show three isomers-two cis and one trans.
  • Tetrahedral geometry does not display these isomers. However, octahedral complexes do show cis and trans isomerism. In complexes with formula [MX2L4] type, we can have the X ligands in the arrangement of cis or trans to each other.
  • We also observe this type of isomerism when bidentate ligands L–L [e.g., NH2 CH2 CH2 NH2 (en)] are present in complexes with [MX2(L–L)2] type formula. There is another type of geometrical isomerism that we find in octahedral coordination entities with [Ma3b3] type formula. An example is [Co(NH3)3(NO2)3].

  • Facial and Meridional isomer:

    • Facial isomers are those in which three donor atom of the same ligand occupies an adjacent position at the corner of an octahedral face. They have the ligand in the cis arrangement.
    • And we get meridional isomer when the position of the ligands are around the meridian of the octahedron. Here the ligands are in the trans arrangement .

    Session Quiz:
    1. The number of possible isomer for the octahedral complex ion [Co(en)Cl2Br2]- is _______ .
    2.What is the number of isomer exist for [Mo(C5H5N)3(CO)3]  ?


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