Question

Which of the following complexes will show optical isomerism?
(A)- ${{[Cr{{(N{{H}_{3}})}_{6}}]}^{2+}}$
(B)- ${{[Ni{{({{H}_{2}}O)}_{6}}]}^{2+}}$
(C)- $[Pt{{(N{{H}_{3}})}_{3}}Br]N{{O}_{3}}$
(D)- $[Cr{{(en)}_{3}}]C{{l}_{3}}$

Answer 1

Optical activity of compounds refers to the ability of a compound to form optical isomers. An optically active coordination compound has optical isomers and a coordinate compound is typically optically inactive and does not have optical isomers.

Complete answer:
-Compounds that have the same chemical formula but different structural or spatial arrangements are called Isomers. Due to the complexity in the formulae of coordination compounds, a variety of bond types, and the number of shapes are possible, many different types of isomerism occurs.
-The phenomenon of isomerism is broadly divided into two types- Structural isomerism; Stereoisomerism.
-The isomerism which arises due to the difference in the structures of coordination compounds is called Structural isomerism. The isomerism which arises due to different spatial orientations of ligand or atoms around the metal ion is called Stereoisomerism.
-Structural isomerism is of again four different types, viz. Ionization isomerism, Hydration isomerism, Linkage isomerism, Coordination isomerism.
-Stereoisomerism is of two types, which are Geometrical isomerism and Optical isomerism.
-Optical isomers are two compounds that have the same chemical formula but are non-superimposable on their mirror images. Optical isomers when placed through plane-polarized light can rotate the plane of polarized light in opposite directions.
-Depending on the direction compounds rotate the plane of polarized light in a polarimeter, we have two different forms of isomers- Dextro isomer, which rotates light to right; Laevo isomer, which rotates light to left.
-For determining whether a given coordination compound will exhibit an optical isomer or not, we can use one of the following two methods.

(i) Method 1: The ‘Mirror image method’
-If the mirror image of the compound can be rotated in such a way that we get the identical image to that of the original molecule, then the molecule is known to be superimposable and has no optical isomers.
-If the mirror image of the compound cannot be rotated in such a way that we get an identical image to that of the original molecule, then the molecule is said to be non-superimposable and has optical isomers.

(ii) Method 2: The ‘Plane of Symmetry Method’
-If in the molecule, a plane of symmetry exists around the central atom, then the molecule is said to be achiral. Achiral molecules do not have optical isomers.
-If in the molecule, a plane of symmetry does not exist around the central atom, then the molecule has a chiral center. Chiral molecules do have optical isomers.

-In conclusion, we can say that optical isomers do not have a plane of symmetry and are non-superimposable on their mirror images.
-Following the above guidelines for the coordination compounds given in the question to identify optically active compounds, we get,
$[Cr{{(en)}_{3}}]C{{l}_{3}}$does not contain plane symmetry, hence is an optically active compound.

So, the correct answer is option D.

Note:
However, there is a lot of significance of stereoisomerism in drug pharmacology. Knowledge of isomerism can help in introducing safer and effective drug alternatives for the drugs which are existing today and have few or more side effects. Many existing drugs have gone chiral switch to offer a more effective and better option in the treatment of diseases with no or lesser side effects. The concept of chirality and its importance as a determinant of the pharmacological properties of drugs have been a part of scientific studies for ages.