The table shows Werner’s formulation of four cobalt(III) chloride–ammonia complexes.
Which two complexes in the table are isomers of each other?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Green and Violet
Isomers are compounds with the same formula but distinct properties. In the table, the green and the violet complexes are both \( \ce{[CoCl2(NH3)4]^+ Cl-} \), that is CoCl3.4NH3, and both are 1:1 electrolytes; yet they have different colours.
Why the other options are incorrect
Option (1) Yellow and Purple — Incorrect. The yellow complex is CoCl3.6NH3 and the purple one is CoCl3.5NH3; they differ in the number of NH3 molecules, so they are not isomers.
Option (2) Purple and Green — Incorrect. The purple complex has five NH3 per cobalt and the green one has four, so their formulas differ.
Option (3) Yellow and Violet — Incorrect. The yellow complex has six NH3 per cobalt and the violet one has four, so their formulas differ.
NCERT (2026-27 edition):
5.1 Werner’s Theory of Coordination Compounds – “…the last two compounds in Table 5.1 have identical empirical formula, CoCl3.4NH3, but distinct properties. Such compounds are termed as isomers.”
Table 5.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes