Stereoisomers are those isomers that have the same molecular formula and chemical bonds but they have different spatial arrangements of atoms. As already mentioned, stereoisomerism involves two types of isomerism viz., geometrical isomerism and optical isomerism. These are discussed below:

1. Geometrical isomerism

Geometrical isomerism arises in heteroleptic complexes due to ligands occupying different positions around the central ion. The ligands occupy positions either adjacent to one another or opposite to one another. These are referred to as cis-form (ligands occupy adjacent positions) and trans- form (ligands occupy opposite positions). This type of isomerism is, therefore, also referred to as cis-trans isomerism. This type of isomerism is very common in coordination compounds. This is due to different coordination numbers varying from 2 to 9, commonly encountered in these compounds.

2. Optical isomerism

There are certain substances that can rotate the plane of polarised light. These are called optically active substances. The isomers which rotate the plane of polarised light equally but in opposite directions are called optically active isomers. These are also called enantiomers or enantiomorphs. The isomer which rotates the plane of polarised light to the right is called dextro rotatory designated as (d) and the one which rotates the plane of polarized light to the left is called laevo rotatory designated as (l). A 1 : 1 equilibrium mixture of d and l isomers gives a net zero rotation and is also called racemic mixture. The d and l isomers are mirror images of each other just as left hand is mirror image of the right hand. These mirror image compounds are non-superimposable on each other and do not possess the plane of symmetry. These optical isomers also possess the property of chirality (handedness). The essential condition for a substance to show optical activity is that the substance should not have a plane of symmetry in its structure. The optical isomers have identical physical and chemical properties. They differ only in the direction in which they rotate the plane of polarised light

Which one of the following complexes exhibits optical isomerism?

cis – diammine dicarbonato cobalt (III) ion

The correct answer is option 2. cis – diammine dicarbonato cobalt (III) ion.

Let us evaluate the complexes one by one to identify which can exhibit optical isomerism. Optical isomerism occurs in chiral molecules, where the molecule and its mirror image are non-superimposable.

1. trans-tetrammine dithiocyanato chromium (III) ion \([Cr(NH_3)_4(NCS)_2]^+\):

In the trans configuration, the two dithiocyanato ligands (NCS) are positioned opposite each other. This symmetry ensures that the complex and its mirror image are superimposable. Therefore, this complex cannot exhibit optical isomerism.

2. cis-diammine dicarbonato cobalt (III) ion \([Co(NH_3)_2(CO_3)_2]^+\):

In the cis configuration, the two identical \( NH_3 \) ligands are adjacent to each other, and the two \( CO_3 \) ligands are also adjacent to each other. This arrangement can create a chiral center because the mirror image of this complex would not be superimposable on the original. Thus, this complex can exhibit optical isomerism.

3. trans-diammine dicarbonato cobalt (III) ion \([Co(NH_3)_2(CO_3)_2]^+\):

In the trans configuration, the two \( NH_3 \) ligands and the two \( CO_3 \) ligands are positioned opposite each other. This symmetrical arrangement means the complex and its mirror image are superimposable. Therefore, this complex cannot exhibit optical isomerism.

4. cis-glycinato platinum (II) \([Pt(gly)_2]\):

In a square planar geometry, the glycinato ligands in the cis configuration can create a chiral center. However, for square planar complexes of platinum(II), the cis arrangement does not typically lead to non-superimposable mirror images. Therefore, this complex does not exhibit optical isomerism.

Based on the above analysis, the complex that exhibits optical isomerism is: cis-diammine dicarbonato cobalt (III) ion (\([Co(NH_3)_2(CO_3)_2]^+\)).

This is because the cis configuration of the ligands allows for the formation of non-superimposable mirror images, resulting in optical isomerism.