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.
2. Optical isomerism
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 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.
In the square planar complex [MABXL] and the octahedral complex [Ma3b3], the ligands A, B, X, L, a and b are all unidentate. What is the total number of geometrical isomers possible for these two complexes taken together?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 5
Geometrical isomers have the same ligands but different relative positions of the ligands around the metal. Count the isomers of each complex and then add them.
Square planar [MABXL]: all four ligands are different. Keep A fixed; the ligand opposite (trans) to A can be B, X or L, and each choice gives a different arrangement. So [MABXL] has 3 isomers — two cis and one trans.
Octahedral [Ma3b3]: the three ‘a’ donor atoms can sit on the corners of one octahedral face (facial, fac) or along a meridian of the octahedron (meridional, mer). So [Ma3b3], such as [Co(NH3)3(NO2)3], has 2 isomers.
Total: 3 + 2 = 5.
NCERT (2026-27 edition):
5.4.1 Geometric Isomerism – “Other square planar complex of the type MABXL (where A, B, X, L are unidentates) shows three isomers-two cis and one trans.”
“Another type of geometrical isomerism occurs in octahedral coordination entities of the type [Ma3b3] like [Co(NH3)3(NO2)3]. If three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral face, we have the facial (fac) isomer. When the positions are around the meridian of the octahedron, we get the meridional (mer) isomer …”