Question

Which of the following has an optical isomer?
(A) ${{[Co{{(N{{H}_{3}})}_{3}}Cl]}^{^{+}}}$
(B) ${{[Co(en){{(N{{H}_{3}})}_{2}}]}^{^{2-}}}$
(C) ${{[Co{{({{H}_{2}}O)}_{4}}(en)]}^{3+}}$
(D) ${{[Co{{(en)}_{2}}{{(N{{H}_{3}})}_{2}}]}^{3+}}$

Answer 1

Draw the expanded structure of the complexes given in the options. Now check for any plane of symmetry. If it does exist, then the complex does not have any optical isomer. The complex having no plane of symmetry will result in having a pair of optical isomers.

Complete step-by-step answer:
The word “isomer” is derived from the Greek words "isos" and "mers". "Isos" means equal and "mers" means parts, so "isomers" means equal parts. Isomerism is the phenomenon in which two or more compounds have the same chemical formula but differ in chemical structures. Chemical compounds that have identical chemical formulas but differ in properties and the arrangement of atoms in the molecule are called isomers i.e. they exhibit isomerism. Isomerism is of two types namely, Structural isomerism and stereoisomerism.
In stereoisomerism, the compounds have the same chemical formula but differ in their respective orientations of the atoms belonging to the compound in a 3D space.
The types of stereoisomerism are:
- Geometrical
- Optical
In the compounds given in options, the complex which has an optical isomer can be ${{[Co{{(en)}_{2}}{{(N{{H}_{3}})}_{2}}]}^{3+}}$.
This is because optical isomers are observed for octahedral complexes and tetrahedral complexes only. So, we can conclude that this is the correct answer even without drawing the complex structure.

Therefore, the correct answer is option (D).

Additional information: In structural isomerism the functional groups and the atoms in the molecules of these isomers are bonded in different ways. Structural isomers have different IUPAC names although their chemical formulae are the same.
The types of structural isomerism are:
- Chain
- Positional
- Functional
- Metamerism
- Tautomerism
- Ring - chain

Note: It is important to know the reason why square planar complexes do not exhibit optical isomerism. All the ligands in a square planar complex are in one plane only. So there even the mirror obtained can be superimposed by rotation about the central metal atom. Thus there is no optical activity observed in a square planar complex.