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

Select correct statement

All are correct

The correct answer is option 4. All are correct.

Let us break down each statement to explain why they are correct:

1. Geometrical isomers may differ in dipole moment and visible/UV spectra:

Geometrical isomers refer to isomers that have the same molecular formula and connectivity but differ in the spatial arrangement of atoms around a double bond or within a ring. This spatial arrangement affects the distribution of charge within the molecule, leading to differences in dipole moment. Additionally, the different spatial arrangements can result in variations in the absorption of light, hence differing in their visible/UV spectra.

2. Complexes of the type \([Ma_3b_3]\) can also have facial (fac) and meridional (mer) isomers:

The notation \([Ma_3b_3]\) refers to octahedral coordination complexes where there are three ligands of one type (a) and three ligands of another type (b) around a central metal atom (M). These complexes can exhibit different geometric isomers based on the spatial arrangement of these ligands around the central metal atom. Facial (fac) isomers have three ligands of one type adjacent to each other on one face of the octahedron, while meridional (mer) isomers have three ligands of one type located at adjacent vertices of the octahedron.

3. Optical isomer exists for the complex trans-\([Co(en)_2Cl_2]^+\):

The complex trans-\([Co(en)_2Cl_2]^+\) refers to a cobalt complex where "en" stands for ethylenediamine. This complex has a central cobalt atom surrounded by two ethylenediamine ligands and two chloride ions. The arrangement of these ligands around the cobalt atom is such that it creates a chiral center. A chiral center is a feature that allows the molecule to exist in two non-superimposable mirror image forms, known as optical isomers.

Therefore, each statement correctly describes a characteristic of chemical isomerism and coordination complexes, making Option 4 ("All are correct") the correct choice.