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Introduction
Compounds having similar chemical formulas but different chemical structures this phenomenon is known as Isomerism. Thus, these compounds having similar chemical formulae and differ in properties and arrangement of atoms in molecules are known as isomers.
Geometrical isomerism is a type of stereoisomerism that has a similar molecular formula and structure but differs in arrangement of atoms. This type of isomerism arises in heteroleptic complexes. Geometrical isomerism arises due to different geometric arrangements of ligands. Geometrical isomerism is also known as Cis-trans isomerism or configurational isomerism.
Types of Geometrical Isomerism
There are two types of geometrical isomerism based on the position of ligands they are discussed below:
- Cis form: in this form two identical ligands occupy the adjacent position.
- Trans form: in this form the two identical ligands are placed opposite to each other.
Geometrical Isomerism in Square Planar
In square planar structures geometrical isomerism occurs only when there is a change in the spatial arrangements of the ligands. Cis and Trans are the most common terms used in these types of isomerism to determine the positions of identical ligands, whether they are placed adjacent or opposite to each other. As seen in example: [Pt(Cl)2(NH3)2].
Geometrical isomerism seen in MA2B2
This type of isomerism arises due to changes in arrangement of ligands. E.g. [Pt(NH3)2Cl2].
Geometrical isomerism seen in MA2BC
Pt[(NH3)Cl(py)2] this compound has two possible types of geometry, the geometry depends on the position of pyridine group , therefore it has a square planar geometry.
Geometrical isomerism in M(AB)2
Examples of this type of Isomerism is seen in [Pt(gly)2]
Geometrical isomerism in MA4B2
The type of isomerism possible for this type is [Cr(NH3)4Cl2]+ the structure possible for this type of isomerism is
Geometrical isomerism in M(AA)2B2
This type of isomerism contains one symmetrical didentate ligand and other two unidentate ligands which may be same or different e.g. [CoCl2(en)2]+ .
Characteristics of Geometrical isomerism in octahedral complexes
Octahedral complexes also have cis trans isomers, die to a change in the spatial arrangement of ligands. In case of cis forms same groups occupies the adjacent position at the corner of one octahedral face this is known as facial or fac-isomer, where as in contrast the region of donor atoms around the meridian of octahedron is known as the meridional or mer-isomer. E.g. [Co(NH3)3Cl3] and [Co(NH3)3(NO2)3]
Conclusion
To get geometric isomers we must have:
- restricted rotation (frequently involving a carbon-carbon double bond for preliminary purposes);
- two different groups on the left-hand and right -end of the bond. It doesn’t matter if the left-hand groups are the same as the right-hand ones or not.
