Question

The octahedral complex of a metal ion $M^{3+} $ with four monodentate ligands $L_1,L_2,L_3 \, and \, L_4 $ absorb wavelengths in the region of red, green, yellow and blue, respectively. The increasing order of ligand strength of the four ligands is

• A. $L_4 < L_3, L_2 < L_1 $
• B. $L_1 < L_3 < L_2 < L_4 $
• C. $L_3 < L_2 < L_4 < L_1 $
• D. $L_1 < L_2 < L_4 < L_3 $

Answer 1

The Correct Option is B

 To determine the increasing order of ligand strength based on the absorption of different wavelengths for the octahedral complex of a metal ion \( M^{3+} \) with four monodentate ligands, we utilize the spectrochemical series. The spectrochemical series arranges ligands in order of their field strength, which correlates with the ability of the metal-ligand complex to absorb certain wavelengths of light.

According to the spectrochemical series, ligands that cause the absorption of light with shorter wavelengths (higher energy) are stronger field ligands compared to those absorbing longer wavelengths (lower energy). Therefore, ligands absorbing blue light are stronger than those absorbing red light.

Given the color of light absorbed by each ligand in the complex:

• \( L_1 \) absorbs red light
• \( L_3 \) absorbs green light
• \( L_2 \) absorbs yellow light
• \( L_4 \) absorbs blue light

The wavelength order for the colors is as follows (approximate wavelengths for visible light):

1. Red: ~620-750 nm
2. Green: ~495-570 nm
3. Yellow: ~570-590 nm
4. Blue: ~450-495 nm

Thus, according to the spectrochemical series and wavelength energies, the order from weaker to stronger ligands is: red < green < yellow < blue.

Therefore, the increasing order of ligand strength for the given ligands is:

\(L_1 < L_3 < L_2 < L_4\)

This matches the correct option provided. Each step follows from the fundamental understanding of how ligands affect the d-orbitals of a metal ion and their position in the spectrochemical series which correlates with absorbed wavelengths.