Question:medium

The correct order of the complexes
[5pt] $[\text{Co(NH}_3)_5(\text{H}_2\text{O})]^{3+}$ (A),
[3pt] $[\text{Co(NH}_3)_6]^{3+}$ (B),
[3pt] $[\text{Co(CN)}_6]^{3-}$ (C),
[3pt] $[\text{CoCl(NH}_3)_5]^{2+}$ (D)
[5pt] in terms of wavelength of light absorbed is:

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Remember the spectrochemical series and how it relates to the strength of ligands and the energy of absorbed light.
Updated On: Jan 14, 2026
  • D $>$ A $>$ B $>$ C
  • C $>$ B $>$ D $>$ A
  • D $>$ C $>$ B $>$ A
  • C $>$ B $>$ A $>$ D
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The Correct Option is A

Solution and Explanation

To order the complexes by absorbed light wavelength, we must consider the crystal field splitting energy (\( \Delta \)) and the ligands present in each complex.

Crystal field theory posits that ligand strength influences d-orbital splitting in transition metal complexes. The spectrochemical series ranks ligand strength. A simplified series is:

  • Strong field ligands: CN- > NO2- > en > NH3
  • Weak field ligands: H2O > OH- > F- > Cl-

Greater ligand field strength results in larger splitting and absorption of shorter wavelengths.

Analysis of each complex:

  1. Complex D: [CoCl(NH3)5]2+
    • Ligands: NH3 (stronger than Cl-), Cl- (weak field ligand)
    • Weakest ligand combination leads to minimal splitting and absorption at the longest wavelength.
  2. Complex A: [Co(NH3)5(H2O)]3+
    • Ligands: NH3, H2O (both stronger than Cl-)
    • Absorbs at a shorter wavelength than D.
  3. Complex B: [Co(NH3)6]3+
    • Ligands: NH3 (strong field ligand)
    • Greater splitting than A, absorbing at an even shorter wavelength.
  4. Complex C: [Co(CN)6]3-
    • Ligands: CN- (very strong field ligand)
    • Maximum field splitting, resulting in absorption at the shortest wavelength.

Therefore, the order from longest to shortest wavelength absorbed is:

D > A > B > C

This corresponds to the option: D $>$ A $>$ B $>$ C

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