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The compounds having same chemical formula but different structural arrangement of their atoms, and hence different physical and chemical properties, are called isomers, and the phenomenon is called isomerism. Isomerism in complexes is of two types:
  1. structural isomerism and
  2. stereoisomerism.

Structural isomerism

This isomerism arises due to the difference in structures of coordination compounds and is of the following types:
  1. Ionization isomerism: Complexes that have the same empirical formula and are produced by the interchange of the position of the ligands inside the complex zone and outside the complex zone are called ionization isomers. They give different ions.
    For example,
    [Co(NH3)4Cl2]NO2 Description: 52722.png [Co(NH3)4Cl2]+ + Description: 50589.png
    [Co(NH3)4Cl(NO2)]Cl Description: 52731.png [Co(NH3)4Cl(NO2)]+ + Cl
    The number of ions in a solution can be determined by conductivity measurement. The more the number of ions in a solution, the more is the conductivity. The greater the change on ions, the greater is the conductivity of solution.
  2. Hydrate isomerism: This type of isomerism arises due to the different position of water molecules inside and outside the coordination sphere. For example,
    1. [Cr(H2O)6]Cl3 (violet) does not lose water over H2SO4 and all Cl ions are immediately precipitated by (Ag+) ions.
    2. [Cr(H2O)5Cl]Cl2 H2O (green) loses H2O over H2SO4 and two Cl ions are precipitated by (Ag+) ions.
    3. [Cr(H2O)4Cl2]Cl 2H2O (green) loses two water molecules over H2SO4 and only one Cl ion is precipitated by Ag+ ions.
  3. Ligand isomerism: Some ligands themselves are capable of existing as isomers; for example, diamino propane can exist both as 1,2-diaminopropane (pn) and 1,3-diaminopropane, also called trimethylenediamine (tn)
    Description: 50596.png
    When these ligands (for example, pn and tn) are associated to form complexes, the complexes are isomers of each other.
  4. Coordination isomerism: If both cation and anion of a complex compound are complex, there may be an exchange of ligands between the two coordination spheres, giving rise to isomers known as coordination isomers. Examples are
    1. [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6] [Co(CN)6]
    2. [Cu(NH3)4 [PtCl4] and [Pt(NH3)4] [[CuCl4]
  5. Linkage isomerism: Those complexes in which the ligands can coordinate with the central metal ion through either of the two atoms give rise to the linkage isomerism.
    The best known ligands of this type are Description: 50603.png, and Description: 50613.png ions. In complexes containing Description: 50624.png ion as ligand, Description: 50634.png ion may attach with the central ion either through O-atom or through N-atom. Examples are
    1. [Co(NH3)5(NO2)]Cl2Pentaamminenitrocobalt(III) chloride.
    2. [Co(NH3)5(ONO)]Cl2Pentaamminenitritocobalt(III) chloride.


Stereoisomers have the same atoms, same sets of bonds, but differ in the relative
orientation of these bonds.
  1. Geometrical isomerism: Geometrical isomerism is due to ligands occupying different positions around the central metal ion. The ligands occupy positions either adjacent to one another or opposite to one another. These are referred to as cis-form and trans-form, respectively. This type of isomerism is, therefore, also referred to as cistrans isomerism.
    1. [Ma2b2] type complexes
      Description: 52740.png
      Examples: [Pt+2(NH3)2Cl2], [Pt2+(NH3)2Br2], and [Pd2+ (NH3)2(NO2)2] are square planar complexes which exhibit cis-trans isomerism.
    2. [Mabcd] type complexes
      Square planar complexes of this type exist in three isomeric forms; for example, [Pt2+(NH3)(Py)(Cl)(Br)] exist in the following structures:
      Description: 52757.png
      [Pt2+(NO2)(Py)(NH3)(NH2OH)]+ and [Pt2+(C2H4)(NH3)(Cl)(Br)] are other examples of square planar complexes which exist in three isomeric forms.
    3. [Ma2bc] type complexes
      Square planar complexes of this type also show cistrans isomerism. For example, [Pd2+Cl2BrI]2– exists in the following cistrans form:
      Description: 52768.png
    4. [Ma4b2]n± type complexes
      Description: 52781.png
      Examples of such complexes are [Co3+(NH3)4Cl2]+, [Co3+(NH3)4(NO2)2]+, etc.
    5. [Ma3b3]n± type complexes
      Description: 52791.png
      When each trio of donor atoms (viz., the ligands a, a, and a) occupy adjacent positions at the corners of an octahedral face, we have facial (fac) isomer. When the positions are around the meridian of the octahedron, we get meridional (mer) isomer.
      Complexes such as [Co3+(NH3)3Cl3], [Cr3+(NH3)3Cl3], [Ru3+(H2O)3Cl3], [Pt4+(NH3)3Br3]+, etc., exhibit such type of isomerism.

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