Question

Homoleptic octahedral complexes of a metal ion 'M³⁺' with three monodentate ligands L₁, L₂ and L₃ absorb wavelengths in the region of green, blue and red respectively. The increasing order of the ligand strength is
1. L₁ < L₂ < L₃
2. L₂ < L₁ < L₃
3. L₃ < L₁ < L₂
4. L₃ < L₂ < L₁

Answer 1

Correct Answer - Option 3 : L₃ < L₁ < L₂

Concept:

Homoleptic Complexes are compounds in which all the ligands bound to the metal center are identical.

Lesser the wavelength of light absorbed more energy greater will be ligand strength.

The ligand strength is classified into two types. They are strong field ligand and weak field ligand. The higher the oxidation state of the metal, the stronger the ligand field that is created. Usually, the field strength of the ligand, which is also, determines whether an octahedral complex is high or low spin. This is where we use the spectrochemical series to determine ligand strength.

In homoleptic complexes, the metal atom/ion is linked to only one type of ligand. Assuming, ligands are neutral, the octahedral complexes of M³⁺ can be,

[M(L₁)₆] ³⁺][M(L₂)₆]³⁺][M(L₃)₆ ]³⁺]

(I)                (II)                  (III)

Green         Blue               Red

So, λL₃ > λL₁ > λL₂

λ is wavelength

L₁, L₂, L₃ - three monodendate ligands.

\({\rm{\Delta }}_{{\rm{absorption}}}^\circ \) of monodendate ligands.

\(\therefore \;{\rm{\Delta }}_{{\rm{absorption}}}^\circ \) : ΔL₂ > ΔL₁ > ΔL₃

We know that, the ligand strength is directly proportional to \({\rm{\Delta }}_{{\rm{absorption}}}^\circ \)

So, the increasing order of the ligand strength will be, L₃ < L₁ < L₂