Compounds having the same molecular formula but different structures or spatial arrangements are called isomers and the phenomenon is referred as isomerism.
(1) Structural isomerism : Here the isomers have different arrangement of ligands around the central metal atom. It is of the following types :
(i) Ionization isomerism :The co-ordination compound having the same composition or molecular formula but gives different ions in solution are called ionization isomers.
There is exchange of anions between the co-ordination sphere and ionization sphere.
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[COBr(NH3)5]SO4
Pentaaminebromo cobalt (III) Sulphate
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[Co SO4(NH3)5]Br
Pentaaminesulphato cobalt (III) bromide
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SO42– present in ionisation sphere |
Br– present in ionization sphere |
Gives white precipitate with BaCl2 |
Gives light yellow precipitate with AgNO3 |
(ii) Co-ordination isomerism : In this case compound is made up of cation and anion and the isomerism arises due to interchange of ligands between complex cation and complex anion.
Example: [Co(NH3)6][Cr(CN)6]
[Cr(NH3)6][Co(CN)6]
hexaamine cobalt (III) hexacyano chromate (III) hexaamine chromium (III) hexacyanocobalt (III)
complex cation contains → NH3 ligand (with cobalt) complex anion contains → NH3 ligand (with chromium) complex anion contains → CN– ligand (with chromium) complex anion contains → CN– ligand (with cobalt)
(iii) Linkage isomerism :In this case isomers differ in the mode of attachment of ligand to central metal ion and the phenomenon is called linkage isomerism.
Example: [Co ONO(NH3)5]Cl2; [Co NO2(NH3)5]Cl2
Pentaamminenitritocobalt (III)
Pentaaminenitrocobalt (III) chloride
:O – NO– oxygen atom donates lone pair of electrons (nitrito) NO2– nitrogen atom donates lone pair of electrons (nitro)
(iv) Hydrate isomerism :Hydrate isomers have the same composition but differ in the number of water molecules present as ligands and the phenomenon is called hydrate isomerism.
Examples :(a) [Cr(H2O)6]Cl3 hexaaquachromium (III) chloride (violet)
(b) [Cr(H2O)5Cl]Cl2.H2O pentaaquachlorochromium (III) chloride monohydrate (blue green)
(c) [Cr(H2O)4Cl]Cl2.2H2O tetraaquadichloro chromium (III) chloride dihydrate (green)
(2) Stereo isomerism or space isomerism : Here the isomers differ only in the spatial arrangement of atoms of groups about the central metal atom. It is of two types :
(i) Geometrical or Cis-trans isomerism : This isomerism arises due to the difference in geometrical arrangement of the ligands around the central atom. When identical ligands occupy positions near to each other called cis-isomer. When identical ligands occupy positions opposite to each other called trans –isomer. It is very common in disubstituted complexes with co-ordination number of 4 and 6.
• Complexes of co-ordination number 4
Tetrahedral geometry : In this case all the four ligands are symmetrically arranged with respect to one another as such geometrical isomerism is not possible.
Square planar geometry : The four ligands occupy position at the four corners and the metal atom or ion is at the center and lie in the same plane.
Type: [Ma2b2], M = Pt, a = Cl, b = NH3
Example: [PtCl (NH3)(Py)2]
Complexes of co-ordination number 6 Octahedral geometry : Here the metal atom or ion lies at the center and 1 to 6 position are occupied by the ligands.
Cis–Positions : 1–2, 2–3, 3–4, 4–5
Trans – position : 1–4, 2–5, 3–6
Type-I: Ma4b2, M = Co, a = NH3, and b = Cl
Example: [CoCl2 (NH3)4]+
ion
Type –II [Ma3b3], M = Rh, a = Cl and b = Py
Example : [RhCl3(Py)3]
Type –III [M(aa)2(en)2]++, M = Co, a – a = CH2NH2
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CH2NH2
(bidentate)
b = Cl (monodentate)
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