The wavelength of light absorbed for the following complexes are in the order. $\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}$; $\left[\mathrm{Co}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]^{3+}$; $\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}$; $\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}\left(\mathrm{H}_{2}\mathrm{O}\right)\right]^{3+}$; $\left[\mathrm{CoF}_{6}\right]^{3-}$ (I) (II) (III) (IV) (V)

(D) III $<$ I $<$ IV $<$ II $<$ V

(A) III $<$ I $<$ II $<$ IV $<$ V

(B) III $<$ I $<$ IV $<$ V $<$ II

(C) III $<$ IV $<$ I $<$ II $<$ V

JEE Main · 2026 · 24 Jan 2026 · Shift 2 · Coordination Compounds

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

[Co (NH_{3})_{6}]^{3 +}

;

[Co (H_{2} O)_{6}]^{3 +}

;

[Co (CN)_{6}]^{3 -}

;

[Co (NH_{3})_{5} (H_{2} O)]^{3 +}

;

[CoF_{6}]^{3 -}

(I) (II) (III) (IV) (V)

Answer: (D)

The wavelength absorbed is inversely proportional to the crystal-field splitting

Δ_{o}

: a stronger-field ligand gives a larger

Δ_{o}

and absorbs at shorter wavelength. For these

C o^{3 +}

complexes the field strength follows the spectrochemical series

F^{-} < H_{2} O < N H_{3} < C N^{-}

, so

Δ_{o}

falls in the order III

(C N^{-})

> I

(N H_{3})

> IV

(5 N H_{3} + H_{2} O

, just below pure

N H_{3})

> II

(H_{2} O)

> V

(F^{-})

. Taking reciprocals, the absorbed wavelengths increase as III

<

<

<

<

JEE Main · 2026 · 24 Jan 2026 · Shift 2 · Single correct option