Three complexes, \[ \left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+}(\mathrm{I}),\left[\mathrm{co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{H}_{2} \mathrm{O}\right]^{3+}(\mathrm{II}) \text { and }\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}(\mathrm{III}) \] Absorb light in the visible region. The correct order of the wavelength of light absorbed by them is

JEE Main · 2019 · 10 Apr 2019 · Shift 1 · Coordination Compounds

Three complexes,

[CoCl (NH_{3})_{5}]^{2 +} (I), [co (NH_{3})_{5} H_{2} O]^{3 +} (II) and [Co (NH_{3})_{6}]^{3 +} (III)

Absorb light in the visible region. The correct order of the wavelength of light absorbed by them is

(A)(II) $>$ (I) $>$ (III)
(B)(III) $>$ (II) $>$ (I)
(C)(I) $>$ (II) $>$ (III)✓ Correct
(D)(III) $>$ (I) $>$ (II)

Answer: (C)

Solution

Crystal field splitting grows with ligand field strength, and the order here is

Cl X^{-} < H X_{2} O < NH X_{3}

. The all-ammine complex has the largest splitting, the aqua complex an intermediate one, and the chloro complex the smallest. A larger splitting means a higher absorbed energy and so a shorter wavelength, which places the absorbed wavelengths as chloro

>

aqua

>

ammine.

JEE Main · 2019 · 10 Apr 2019 · Shift 1 · Single correct option

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