Question:medium

Correct increasing order for the wavelengths of absorption in the visible region for the complexes of Co3+ is

Updated On: Apr 22, 2026
  • [Co(en)3]3+, [Co(NH3)6]3+, [Co(H2O)6]3+
  • [Co(H2O)6]3+, [Co(en)3]3+, [Co(NH3)6]3+
  • [Co(H2O)6]3+, [Co(NH3)6]3+, [Co(en)3]3+
  • [Co(NH3)6]3+, [Co(en)3]3+, [Co(H2O)6]3+
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The Correct Option is A

Solution and Explanation

The problem requires identifying the correct increasing order of wavelengths for the absorption in the visible region for the complexes of Co3+. This can be determined by considering the strength of the ligands attached to the cobalt ion, which in turn affects the splitting of d-orbitals (crystal field splitting) and thus the wavelength of light absorbed.

The relevant complexes given are: 

  • [Co(en)3]3+
  • [Co(NH3)6]3+
  • [Co(H2O)6]3+

Here is how to approach the problem:

  1. The Crystal Field Theory tells us that the splitting of d-orbitals depends on the strength of the ligand. Stronger field ligands create larger splitting. In this context, the crystal field strength order among common ligands is generally: en > NH3 > H2O.
  2. The spectrochemical series provides the order of ligands in terms of their field strength. For our complexes, it would be:
    • en (ethylenediamine) - strong field
    • NH3 (ammonia) - intermediate field
    • H2O (water) - weak field
  3. The greater the field strength, the larger the energy splitting, and the shorter the wavelength absorbed. Thus, the order of increasing wavelength absorbed is from strongest to weakest field ligand.

Based on this reasoning, the order in which the wavelength absorbed increases (i.e., lowest to highest) is:

  • [Co(en)3]3+ (highest energy, shortest wavelength)
  • [Co(NH3)6]3+ (intermediate energy and wavelength)
  • [Co(H2O)6]3+ (lowest energy, longest wavelength)

Therefore, the correct increasing order for the wavelengths of absorption in the visible region is: 
[Co(en)3]3+, [Co(NH3)6]3+, [Co(H2O)6]3+

This solution takes into account the field strength of the ligands affecting the d-orbital splitting and hence the wavelength absorbed by each complex.

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