JEE Main 2026 — Jan 24, Shift 2 — Chemistry solution

JEE Main2026Jan 24, Shift 2Chemistry

The wavelength of light absorbed for the following complexes are in the order.

(I) $[Co (NH_{3})_{6}]^{3 +}$ ; (II) $[Co (H_{2} O)_{6}]^{3 +}$ ; (III) $[Co (CN)_{6}]^{3 -}$ ;

(IV) $[Co (NH_{3})_{5} (H_{2} O)]^{3 +}$ ; (V) $[CoF_{6}]^{3 -}$

A
III $<$ I $<$ II $<$ IV $<$ V
B
III $<$ I $<$ IV $<$ V $<$ II
C
III $<$ IV $<$ I $<$ II $<$ V
D
III $<$ I $<$ IV $<$ II $<$ V

Solution

Wavelength of absorption is inversely related to crystal field splitting energy (CFSE): stronger field $\Rightarrow$ larger $Δ_{o}$ $\Rightarrow$ shorter wavelength absorbed.

Ligand field strength (spectrochemical order):

$F^{-} < H_{2} O < NH_{3} < CN^{-}$

So CFSE ordering: V $<$ II $<$ IV $<$ I $<$ III (IV is mixed, between II and I).

Wavelength absorbed (inverse of CFSE): III $<$ I $<$ IV $<$ II $<$ V.

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