The type of isomerism exhibited by the complex \([CoCl_2(en)_2]\) is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Geometrical isomerism
\( \ce{[CoCl2(en)2]} \) is an octahedral complex of the type [M(AA)2B2]. The two \( \ce{Cl^{-}} \) ligands can be placed adjacent (cis) or opposite (trans) to each other, so it shows geometrical (cis–trans) isomerism. (The cis form is also chiral and exists as d and l forms.)
Why the other options are incorrect
Option (1) Coordination isomerism — Incorrect. Needs both a complex cation and a complex anion that can exchange ligands, as in \( \ce{[Co(NH3)6][Cr(CN)6]} \); here there is only one metal centre.
Option (2) Ionization isomerism — Incorrect. Needs a counter ion outside the coordination sphere that can swap places with a ligand; the formula given has no counter ion.
Option (3) Linkage isomerism — Incorrect. Needs an ambidentate ligand such as \( \ce{NO2^{-}} \) or \( \ce{SCN^{-}} \); \( \ce{Cl^{-}} \) and en bind through only one kind of donor atom.
NCERT (2026-27 edition):
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.”