Which of the following types of octahedral complexes does not show geometrical isomerism? (A and B are unidentate ligands)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → MA5B
Geometrical isomerism needs at least two different ways of placing the ligands in space, such as cis/trans or fac/mer. All six positions of an octahedron are alike: each has four neighbours at 90° (cis) and one opposite at 180° (trans).
Option (1) MA3B3 — Shows geometrical isomerism. The three A can sit on one face of the octahedron (fac) or around a meridian (mer): 2 isomers.
Option (2) MA4B2 — Shows geometrical isomerism. The two B can be adjacent (cis) or opposite (trans): 2 isomers.
Option (3) MA5B — Does not show geometrical isomerism. There is only one B. Wherever it sits, it has four A cis to it and one A trans to it, so every placement is the same arrangement: 1 form only.
Option (4) MA2B4 — Shows geometrical isomerism. The two A can be cis or trans to each other: 2 isomers.
NCERT (2026-27 edition):
5.4.1 Geometric Isomerism – “This type of isomerism arises in heteroleptic complexes due to different possible geometric arrangements of the ligands. Important examples of this behaviour are found with coordination numbers 4 and 6.”