Geometric Isomers
The Cis and Trans forms of the tetraamminedichlorocobalt(III) ion
Stereoisomerism occurs with the same bonds in different orientations relative to one another. Stereoisomers have the same chemical bonds but different spatial arrangements.Stereoisomers can be classified as Geometric isomers and Optical isomers.
Geometric Isomers:
Isomerism in which the arrangement of constituent atoms is different though the same bonds are present such an isomerism is called as Geometrical isomerism. Geometric isomers are Cis and trans isomers.
Cis Platin and Trans Platin
Cis – Isomer: Isomer in which the like ligands are adjacent in position is called as Cis isomer.
Trans Isomer: Isomer in which the like ligands are across in position is called as trans isomer .
Cis and Trans Isomers are possible for both square planar and octahedral complexes, but not tetrahedral
A number of examples of these types have been isolated and characterised and they show very different chemical and biological properties.
Example: [Pt(NH3)2Cl2]
The isomeric pair of diamminedichloroplatinum compounds
The two ammonia molecules and the two chlorine atoms are situated next to one another in one isomer, called the Cis isomer, and across from one another in the other, the trans isomer.
Cis – PtCl2(NH3)2 is an anti–cancer agent (Cisplatin)
whereas the trans – isomer is inactive against cancer (it is toxic).
Optical isomers :
Optical isomers are mirror images of each other and have chiral centers. If a molecule and its mirror image are not superimposable they are said to be chiral and they are said to be related to one another as enantiomers.
Enantiomers:
Enantiomers exist when the molecules of the substances are mirror images but are not superimposable upon one another.
In coordination compounds, enantiomers can arise either from the presence of an asymmetric ligand, such as one isomer of the amino acid, alanine (aminopropionic acid)or from an asymmetric arrangement of the ligands.
Familiar examples
Octahedral complexes carrying three didentate ligands, such as ethylenediamine, NH2CH2–CH2NH2.
The two enantiomers corresponding to such a complex are depicted by the structures.
The ethylenediamine ligands above are indicated by a curved line between the symbols for the nitrogen atoms.
Diastereomers:
Diastereomers on the other hand, are not superimposable and also are not mirror images.
Using AB as an example of a chelating ligand, in which the symbol AB implies that the two ends of the chelate are different, there are six possible isomers of a complex Cis – [M(AB)2X2].
Properties of Optical Isomers
Optical isomers have similar physical and chemical properties but differ in rotating the plane of plane polarized light.
Isomer which rotates the plane polarized light to the right is called dextro rotatory (d–form) and the isomer which rotates the plane polarized light to the left is called laevorotatory (l–form)