Chemistry · Coordination Compounds

JEE Advanced 2025 — Paper 1 — Question 32

The correct order of the wavelength maxima of the absorption band in the ultraviolet-visible region for the given complexes is

Option A:
$\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}\left(\mathrm{H}_{2} \mathrm{O}\right)\right]^{3+}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}(\mathrm{Cl})\right]^{2+}$
Correct
Option B:
$\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}(\mathrm{Cl})\right]^{2+}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}\left(\mathrm{H}_{2} \mathrm{O}\right)\right]^{3+}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}<\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}$
Option C:
$\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}(\mathrm{Cl})\right]^{2+}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}\left(\mathrm{H}_{2} \mathrm{O}\right)\right]^{3+}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}$
Option D:
$\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}<\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}(\mathrm{Cl})\right]^{2+}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}\left(\mathrm{H}_{2} \mathrm{O}\right)\right]^{3+}$

Answer: A

Step-by-step solution

$\quad \Delta_{\mathrm{O}} \propto \frac{1}{\lambda}$

$\therefore$ The absorb wave length order is

$\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{H}_{2} \mathrm{O}\right]^{3+}<\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}\right]^{2+}$

Answer key and solution verified before publishing.

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Exam: JEE Advanced 2025
Paper: Paper 1
Subject: Chemistry
Chapter: Coordination Compounds
Topic: Theories of Bonding in Coordination Compounds