The correct order of the wavelength of light absorbed by the following complexes is, A. [Co(NH3)6]3+ B. [Co(CN)6]3- C. [Cu(H2O)4]2+ D. [Ti(H2O)6]3+ Choose the correct answer from the options given below:

The correct answer is Option 2. Full mathematical solution steps and hints are accessible in the interactive StemCET workspace.

Chemistry Q57 – NEET-UG 2025

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Home Chemistry Coordination CompoundsQuestion #331

The correct order of the wavelength of light absorbed by the following complexes is, A.

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

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

[C u (H_{2} O)_{4}]^{2 +}

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

Choose the correct answer from the options given below:

Clue 1 of 3: Crystal Field Theory

The color of transition metal complexes arises from the absorption of light, which promotes electrons between d-orbitals split by the ligand field.

🧠 Full Solution Path

● Video Walkthrough

Step 1: Determine Oxidation States and Ligand Strengths✦ Active

First, identify the central metal ion's oxidation state and the nature of the ligand for each complex:

A. $[C o (N H_{3})_{6}]^{3 +}$ : $C o^{3 +}$ , $N H_{3}$ (strong field ligand)

B. $[C o (C N)_{6}]^{3 -}$ : $C o^{3 +}$ , $C N^{-}$ (very strong field ligand)

C. $[C u (H_{2} O)_{4}]^{2 +}$ : $C u^{2 +}$ , $H_{2} O$ (weak field ligand)

D. $[T i (H_{2} O)_{6}]^{3 +}$ : $T i^{3 +}$ , $H_{2} O$ (weak field ligand)

💡 Teacher's Secret Hint

Recall the spectrochemical series to rank ligand field strengths: $C N^{-} > N H_{3} > H_{2} O$ .

Step 2: Rank Complexes by Crystal Field Splitting Energy (

Δ_{o}

)○ Expand

The crystal field splitting energy ( $Δ_{o}$ ) depends on the ligand strength and the oxidation state of the metal ion. Stronger ligands and higher oxidation states lead to larger $Δ_{o}$ .

1. Comparing B and A: Both have $C o^{3 +}$ , but $C N^{-}$ is a stronger ligand than $N H_{3}$ . Thus, $Δ_{o} (B) > Δ_{o} (A)$ .

2. Comparing D and C: Both have $H_{2} O$ ligands, but $T i^{3 +}$ has a higher oxidation state (+3) than $C u^{2 +}$ (+2). Higher oxidation state generally leads to larger $Δ_{o}$ . Thus, $Δ_{o} (D) > Δ_{o} (C)$ .

Combining these, the overall order of $Δ_{o}$ is: $Δ_{o} (B) > Δ_{o} (A) > Δ_{o} (D) > Δ_{o} (C)$ .

💡 Teacher's Secret Hint

Remember that $Δ_{o}$ increases with increasing ligand field strength and increasing metal ion charge.

Step 3: Determine the Order of Absorbed Wavelength (

λ

)○ Expand

The wavelength of light absorbed ( $λ$ ) is inversely proportional to the crystal field splitting energy ( $Δ_{o}$ ), i.e., $λ \propto 1 / Δ_{o}$ .

Therefore, the order of absorbed wavelength will be the inverse of the $Δ_{o}$ order:

C > D > A > B

Expressed in increasing order of wavelength, this is: $B < A < D < C$ .

This matches option (2).

💡 Teacher's Secret Hint

A larger $Δ_{o}$ means higher energy absorption, which corresponds to a shorter wavelength.

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Chemistry Question 57 – NEET-UG 2025

Clue 1 of 3: Crystal Field Theory

🧠 Full Solution Path