Question

The wavelength of light absorbed for the following complexes are in the order
[Co(NH$_3$)$_6$]$^{3+}$ (I), [Co(H$_2$O)$_6$]$^{3+}$ (II), [Co(CN)$_6$]$^{3-}$ (III), [Co(NH$_3$)$_5$(H$_2$O)]$^{3+}$ (IV), [CoF$_6$]$^{3-}$ (V)

• A. III < I < IV < II < V
• B. III < I < II < IV < V
• C. III < IV < I < II < V
• D. III < I < IV < V < II

Hint:

Stronger ligands cause larger crystal field splitting and shorter wavelength absorption.

Answer 1

The Correct Option is A

The problem involves determining the order of wavelengths of light absorbed by different cobalt complexes. This order is determined by the spectrochemical series, which ranks ligands based on their field strength. Strong field ligands cause a large splitting in the d-orbital energies of transition metals, leading to shorter wavelengths (higher energy) being absorbed. Conversely, weak field ligands cause smaller splitting, leading to longer wavelengths (lower energy) absorption.

Let's analyze each complex:

1. [Co(CN)$_6$]$^{3-}$ (III): Cyanide (CN$^-$) is a strong field ligand, causing a large crystal field splitting. This leads to the absorption of shorter wavelengths.
2. [Co(NH$_3$)$_6$]$^{3+}$ (I): Ammonia (NH$_3$) is a moderate field ligand, which causes moderate crystal field splitting.
3. [Co(NH$_3$)$_5$(H$_2$O)]$^{3+}$ (IV): This complex has both ammonia and water ligands. Water (H$_2$O) is a weaker field ligand compared to ammonia, resulting in slightly less splitting compared to [Co(NH$_3$)$_6$]$^{3+}$.
4. [Co(H$_2$O)$_6$]$^{3+}$ (II): Water is a weaker field ligand, causing smaller splitting than NH$_3$.
5. [CoF$_6$]$^{3-}$ (V): Fluoride (F$^-$) is a weak field ligand, causing minimal splitting. Thus, it absorbs light at longer wavelengths.

The order based on the strength of the ligands (from strong field to weak field) and the corresponding order of absorbed wavelengths is:

• III (\([\text{Co(CN)}_6]^{3-}\)): Strong field, absorbs shorter wavelengths.
• I (\([\text{Co(NH}_3)_6]^{3+}\)): Moderate field, absorbs slightly longer wavelengths than CN$^-$.
• IV (\([\text{Co(NH}_3)_5(\text{H}_2\text{O})]^{3+}\)): Even weaker than ammonia alone due to the weaker H$_2$O ligand presence.
• II (\([\text{Co(H}_2\text{O})_6]^{3+}\)): Absorbs longer wavelengths than NH$_3$ complexes due to water's weaker ligand field.
• V (\([\text{CoF}_6]^{3-}\)): Weak field, absorbs the longest wavelengths.

Hence, the correct order of wavelengths absorbed is: III < I < IV < II < V.