The correct order of the complexes [Co(NH3)5(H2O)]3+ (A), [Co(NH3)6]3+ (B), [Co(CN)6]3- (C) and [CoCl(NH3)5]2+ (D) in terms of wavelength of light absorbed is

The correct answer is Option 4. Full mathematical solution steps and hints are accessible in the interactive StemCET workspace.

Chemistry Q63 – JEE-MAIN 2025

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Home Chemistry Coordination CompoundsQuestion #343

The correct order of the complexes

[C o (N H_{3})_{5} (H_{2} O)]^{3 +}

(A),

[C o (N H_{3})_{6}]^{3 +}

(B),

[C o (C N)_{6}]^{3 -}

[C o C l (N H_{3})_{5}]^{2 +}

(D) in terms of wavelength of light absorbed is

Clue 1 of 3: Spectrochemical Series

The wavelength of light absorbed by a complex is inversely proportional to the crystal field splitting energy ( $Δ_{o}$ ).

🥷

Ninja Strategy•Extreme Ligand Strength Analysis

Identify the complexes with the strongest ( $C N^{-}$ in C) and weakest ( $C l^{-}$ in D) ligand fields. The weakest field (D) will absorb the longest wavelength, and the strongest field (C) will absorb the shortest wavelength. This immediately eliminates options that place C at the longest wavelength end or D at the shortest wavelength end, and helps narrow down the choices.

🧠 Full Solution Path

Step 1: Determine Oxidation State and Ligand Field Strength✦ Active

All complexes have $C o^{3 +}$ as the central metal ion. The spectrochemical series for the ligands involved, from weakest to strongest field, is $C l^{-} < H_{2} O < N H_{3} < C N^{-}$ . The energy of light absorbed ( $E_{a b s}$ ) is directly proportional to the crystal field splitting energy ( $Δ_{o}$ ), and inversely proportional to the wavelength of light absorbed ( $λ_{a b s}$ ). Thus, a weaker ligand field leads to smaller $Δ_{o}$ and longer $λ_{a b s}$ .

Step 2: Order by Crystal Field Splitting Energy (

Δ_{o}

)○ Expand

Based on the ligand field strength, we can order the complexes by their $Δ_{o}$ values:

[C o C l (N H_{3})_{5}]^{2 +} (D) < [C o (N H_{3})_{5} (H_{2} O)]^{3 +} (A) < [C o (N H_{3})_{6}]^{3 +} (B) < [C o (C N)_{6}]^{3 -} (C)

This order reflects the increasing average ligand field strength: $C l^{-}$ is the weakest, followed by $H_{2} O$ , then $N H_{3}$ , and finally $C N^{-}$ is the strongest.

Step 3: Order by Wavelength of Light Absorbed (

λ_{a b s}

)○ Expand

Since $λ_{a b s} \propto 1 / Δ_{o}$ , the order of wavelength absorbed will be the inverse of the $Δ_{o}$ order:

λ_{a b s} (D) > λ_{a b s} (A) > λ_{a b s} (B) > λ_{a b s} (C)

This corresponds to option 4.

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Chemistry Question 63 – JEE-MAIN 2025

Clue 1 of 3: Spectrochemical Series

🧠 Full Solution Path