isomerism of Coordination Compounds

Two or more different compounds having the same formula are called isomers. Two principal types of isomerism are known among coordination compounds. Each of which can be further subdivided.

1.Structural Isomerism.

a) Coordination isomerism
b) Ionisation isomerism
c) Hydrate isomerism
d) Linkage isomerism

2.Stereoisomerism.

a) Geometrical isomerism
b) Optical isomerism

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1.Structural Isomers

There are several types of this isomerism frequently encountered in coordination chemistry and the following represents some of them.

• a) Coordination isomerism: where compounds containing complex anionic and cationic parts can be thought of as occurring by interchange of some ligands from the cationic part to the anionic part.

one isomer [Co(NH₃)₆] [Cr(C₂O₄)₃]
another isomer [Co(C₂O₄)₃] [Cr(NH₃)₆]

• b) Ionisation isomers: where the isomers can be thought of as occurring because of the formation of different ions in solution.

one isomer [PtBr(NH₃)₃]NO₂ -> NO₂- anions in solution
another isomer [Pt(NO₂)(NH₃)₃]Br -> Br- anions in solution

Notice that both anions are necessary to balance the charge of the complex, and that they differ in that one ion is directly attached to the central metal but the other is not. A very similar type of isomerism results from replacement of a coordinated group by a solvent molecule (Solvate Isomerism). In the case of water, this is called Hydrate isomerism.

• c) Hydrate isomerism: the best known example of this occurs for chromium chloride "CrCl₃.6H₂O" which may contain 4, 5, or 6 coordinated water molecules.

[CrCl₂(H₂O)₄]Cl.2H₂O bright-green
[CrCl(H₂O)₅]Cl₂.H₂O grey-green
[Cr(H₂O)₆]Cl₃ violet

These isomers have very different chemical properties and on reaction with AgNO₃ to test for Cl- ions, would find 1, 2, and 3 Cl- ions in solution respectively.

• d) Linkage isomerism: occurs with ambidentate ligands. These ligands are capable of coordinating in more than one way. The best known cases involve the monodentate ligands SCN- / NCS- and NO₂^- / ONO^-.

For example:

[Co(ONO)(NH₃)₅]Cl the nitrito isomer -O attached
[Co(NO₂)(NH₃)₅]Cl the nitro isomer - N attached.

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2.Stereoisomerism

In stereochemistry, stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientations of their atoms in space. Wikipedia

• a) Geometrical isomerism or Cis-Trans isomers:Geometric isomers are two or more coordination compounds which contain the same number and types of atoms, and bonds (i.e., the connectivity between atoms is the same), but which have different spatial arrangements of the atoms.
  
  Not all coordination compounds have geometric isomers.
  
  For example, in the square planar molecule, Pt(NH3)2Cl2, the two ammonia ligands (or the two chloride ligands) can be adjacent to one another or opposite one another.
  
  Note that these two structures contain the same number and kinds of atoms and bonds but are non-superimposable. The isomer in which like ligands are adjacent to one another is called the cis isomer. The isomer in which like ligands are opposite one another is called the trans isomer.
  
  For the common structures which contain two or more different ligands, geometric isomers are possible only with square planar and octahedral structures (i.e., geometric isomers cannot exist for linear and tetrahedral structures).
  
  cis-[Co(NH3)4Cl2]+
  Note that the two chloride ligands are adjacent to one another in this octahedral complex ion. In aqueous solution, this complex ion has a violet color.
  
  trans-[Co(NH3)4Cl2]+
  Note that the two chloride ligands are opposite one another in this complex ion. In aqueous solution, this complex ion has a green color.

• b) Optical isomerism:Optical Isomers
  Optical isomers are two compounds which contain the same number and kinds of atoms, and bonds (i.e., the connectivity between atoms is the same), and different spatial arrangements of the atoms, but which have non-superimposable mirror images. Each non-superimposable mirror image structure is called an enantiomer. Molecules or ions that exist as optical isomers are called chiral.
  
  Not all coordination compounds have optical isomers.
  
  The Two Enantiomers of CHBrClF
  Note that the molecule on the right is the reflection of the molecule on the left (through the mirror plane indicated by the black vertical line). These two structures are non-superimposable and are, therefore, different compounds. 
  
  Pure samples of enantiomers have identical physical properties (e.g., boiling point, density, freezing point). Chiral molecules and ions have different chemical properties only when they are in chiral environments.
  
  Optical isomers get their name because the plane of plane-polarized light that is passed through a sample of a pure enantiomer is rotated. The plane is rotated in the opposite direction but with the same magnitude when plane-polarized light is passed through a pure sample containing the other enantiomer of a pair.