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  Structural isomerism

Structural isomerism:

a) Coordination isomerism:

In bimetallic complex, both complex cation and anion might be present. In such a condition the distribution of ligands among the two coordination spheres can differ, giving mount to isomers termed as coordination isomers. This phenomenon is termed as coordination isomerism. This isomerism is explained by the following pairs of complexes where the complex cation and anion have different metal centers.

1. [CoIII(NH3)6] [Cr(CN)6]   and   [CrIII(NH3)6] [CoIII(CN)6]
Hexammine      hexacyano          Hexamine     hexacyano
cobalt(III)         chromate(III)      chromium (III) cobaltate (III)

2. [PtII(NH3)4] [CuCl4]   and [Cu(NH3)4] [PtCl4]
Tetraammine  Tetrachloro    Tetraammine Tetrachloro
platinum (II)    cuparate (II)     copper (II)   platinate (II)

b) Ionisation isomerism:

Coordination compounds containing similar molecular formula though forming different ions in solution are termed as ionisation isomers. This property is termed as ionisation isomerism.

An illustration of this kind of isomerism is furnished by the red-violet,

[Co(NH3)5Br]SO4                                     [Co(NH3)5SO4]Br
pentaamminebromocobalt(III) sulphate     pentaamminesulphatocobalt (III) bromide


The red-violet isomer outcomes sulphate ion and the red isomer furnishes bromide ion in the solution.

[Co(NH3)4Cl2]NO2             and                  [Co(NH3)4NO2Cl]Cl
Tetraamminedichlorocobalt(III) nitrite       Tetraamminechloronitrocobalt(III) chloride

[Co(NH3)5NO3]SO4            and                   [Co(NH3)5SO4]NO3
pentaamminenitratocobalt(III) sulphate       pentaamminesulphatocobalt(III) nitrate

c) Hydrate isomerism or Solvate isomerism:

The best known illustration of this kind of isomerism takes place for chromium chloride “CrCl3.6H2O” that might contain 4, 5, or 6 coordinated water molecules.

1. [Cr(H2O)4Cl2]Cl.2H2O - Bright green
Tetraaquadichlorochromium (III) chloride dihydrate

2. [Cr(H2O)5Cl]Cl2.H2O - grey-green
Pentaaquachlorochromium (III) chloride monohydrate

3. [Cr(H2O)6]Cl3- Violet
Hexaaquachromium(III) chloride

Such isomers have very dissimilar chemical properties and on reaction with AgNO3 to test for Cl- ions, would get 1,2, and 3 Cl- ions in solution correspondingly.

d) Linkage isomerism:

Linkage isomerism takes place with ambidentate ligands. Such ligands are capable of coordinating in more than one manner. The best known cases include the monodentate ligands SCN- or NCS- and NO2- or ONO-

e) Ligand isomerism:

Ligand isomerism occurs from the presence of ligands that can adopt various isomeric forms. An illustration is given by diaminopropane, that might have the amine groups in the terminal (1,3-) positions or in the 1,2-positions.

380_ligand isomerism.jpg

 

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