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 will be able to show cis–trans isomerism?

all

The answer is 4. all.

Cis–trans isomerism is a type of geometrical isomerism that occurs when two identical ligands are bonded to the same metal ion, but on opposite sides of the metal ion. This type of isomerism is only possible if the two ligands are different.

The complexes Ma₂b₂, M(AB)₂, and Mabcd can all show cis–trans isomerism because they all have two identical ligands bonded to the same metal ion.

The complex Ma₂b₂ can have the two a ligands bonded to the metal ion on opposite sides, giving rise to the cis isomer, or they can be bonded to the metal ion on the same side, giving rise to the trans isomer.

The complex M(AB)₂ can have the two A ligands bonded to the metal ion on opposite sides, giving rise to the cis isomer, or they can be bonded to the metal ion on the same side, giving rise to the trans isomer.

The complex Mabcd can have the two b ligands bonded to the metal ion on opposite sides, giving rise to the cis isomer, or they can be bonded to the metal ion on the same side, giving rise to the trans isomer.

Therefore, all three complexes can show cis–trans isomerism. So the answer is (4).