Question

Consider the following complexes: A=${{[FeC{{l}_{6}}]}^{4-}}$ , B=${{[Fe{{({{H}_{2}}O)}_{6}}]}^{2+}}$ and C=${{[Fe{{({{H}_{2}}O)}_{6}}]}^{3+}}$
The correct order of decreasing wavelength of light absorbed for the following complexes is
(1) C > B > A
(2) A > C > B
(3) B > A > C
(4) A > B > C

Answer 1

Hint: Answer to this question is based on the concept of molecular orbital theory of octahedral complexes and that includes the absorption of light depending on the presence of weak field and strong field ligands.

Complete step by step answer:
In our lower classes, we have studied about the concepts of octahedral and tetrahedral splitting of the energy that is based on the ligands present and also coordination of the ligands that is whether tetrahedral or octahedral.
Let us now see what does the splitting means and how absorption is taken into consideration.
- Octahedral splitting occurs in the octahedral complexes where six sigma donor ligands donate two electrons each to the d – orbital of the central metal atom.
- Based on the molecular orbital energy diagram of sigma bonding in octahedral complexes, we can identify the compound having more energy and thus can find the wavelength of absorption as energy is inversely proportional to the wavelength that is $E\propto \dfrac{1}{\lambda }$
- Now in both ${{[FeC{{l}_{6}}]}^{4-}}$ and ${{[Fe{{({{H}_{2}}O)}_{6}}]}^{2+}}$, the oxidation state of iron in this is +2 and thus here the absorption of ${{[FeC{{l}_{6}}]}^{4-}}$ will be less because energy released here is more as chlorine is weak field ligand compared to that of water. Thus, energy of ${{[Fe{{({{H}_{2}}O)}_{6}}]}^{2+}}$ is more than ${{[FeC{{l}_{6}}]}^{4-}}$
And thus, the wavelength of absorption is higher for the complex A = ${{[FeC{{l}_{6}}]}^{4-}}$ than the complex B =${{[Fe{{({{H}_{2}}O)}_{6}}]}^{2+}}$
In the complex ${{[Fe{{({{H}_{2}}O)}_{6}}]}^{3+}}$, the oxidation state of iron is +3 and thus more the oxidation number, more is the energy released and thus more will be the light absorbed and thus this complex has lesser absorption.
Therefore, the decreasing order of absorption is A > B > C

Thus, the correct answer is option D) A > B > C.

Note: The field strength of the ligands is given by the series known as the spectrochemical series which is based on the increasing order of the ligand field strength and according to this carbonyl ligand that is $CO$ is the strongest field ligand and iodine that is ${{I}^{-}}$ is the weakest field ligand. This series has to be memorised for solving this type of questions.