Isomers are......
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Compounds with same chemical formula but a different arrangement of atoms.
In coordination compounds there are two main kinds of isomerism:
Structural isomerism (different bonds): linkage isomerism, e.g. –\( \ce{NO2} \) and –ONO; coordination isomerism; ionisation isomerism, e.g. \( \ce{[Co(NH3)5SO4]Br} \) and \( \ce{[Co(NH3)5Br]SO4} \); solvate (hydrate) isomerism.
Stereoisomerism (same bonds, different arrangement in space): geometrical (cis/trans, fac/mer) and optical isomerism.
Why the other options are incorrect
Option (2) Heteroleptic complexes only. — Incorrect. A heteroleptic complex is one with more than one kind of ligand, such as \( \ce{[Co(NH3)4Cl2]^{+}} \). Isomerism is not limited to them: the homoleptic \( \ce{[Co(en)3]^{3+}} \) shows optical isomerism.
Option (3) Phenomena that arises in tetrahedral complexes only — Incorrect. Isomers are compounds, not a phenomenon. Isomerism is actually more common in octahedral and square planar complexes; tetrahedral complexes do not show geometrical isomerism.
Option (4) Compounds with different molecular formula but similar physical properties — Incorrect. Isomers must have the same formula, and they usually differ in properties.
NCERT (2026-27 edition):
Isomerism in Coordination Compounds
