What is optical isomerism?
Optical isomers is also called enantiomers and these are the pair of molecules or ions that are non- superimposable mirror images of each other.
What does the term superimposable mean?
The term superimposable means that if one structure is laid over the other of the same molecule the position of all the atoms should be matched and these two are called non-superimposable mirror image of each other.
For example, if a pipette is placed in front of a mirror, the image reflected on the mirror is identical to the pipette itself. So, in this case, we say that pipette and its mirror image are superimposable with each other.
If the left hand is placed in front of a mirror, the image reflected on the mirror will look like the right hand. Thus we can say that left and right hand are the mirror image of each other but are non-superimposable when the left hand is placed over the right hand keeping the palms down, they do not match. This superimposable property of left and right hand is called handedness.
The optical isomers have handedness and are said to be chiral.
What are chiral molecules?
The molecules which are optically active and rotate the plane polarized light towards left or right are known as chiral molecules.
If the plane polarized light is rotated to the right, the isomer is said to be dextrorotatory(d or +) and if it is rotated to the left, the isomer is said to be levorotatory(l or -).The d- and l- isomer of a chiral substance are called enantiomers.
An equimolar mixture of d- and l- isomer, called a racemic mixture.
The essential condition for a substance to be chiral(or optically active) is the substance must have non-superimposable mirror image and it don’t have any plane of symmetry.
Optical isomerism in the square planar complex:
Why square planar compound does not show optical isomerism?
Square planar complex rarely show optical isomerism whether all the four ligands are different are same because they have all the four ligands and the metal cation in the same plane and hence have a plane of symmetry.
How ever there are exceptionally some complexes which exhibit optical isomerism i.e. (isobutylenediamine) (meso-diphenylethylenediamine) palladium (II) or palladium(II) complex.