Chapter 20.4, Problem 1RC

Interpretation Introduction

Interpretation: The colour of the octahedral complexes such as ${\text{CrX}}_{\text{6}}^{\text{3+}}$ and ${\text{CrY}}_{\text{6}}^{\text{3+}}$ are being different.  From this information, the stronger field ligand has to be found out among the neutral ligands $\text{X}$ and $\text{Y}$.

Concept Introduction:

Strong field ligands:

• Gives larger splitting.
• So, $\text{splitting}\text{energy}{\text{Δ}}_{\text{o}}$ will be large.
• As a consequence electrons do not get transmitted to the higher energy level due to the large energy gap in between. So, remains paired up in the lower energy level.
• Thus, results in low-spin complexes.
• The wavelength of absorption corresponding to the large $\text{splitting}\text{energy}{\text{Δ}}_{\text{o}}$ will be of larger wavelength.
• Hence, the wavelength of absorption has a low frequency and lower energy.

Weak field ligands:

• Gives smaller splitting.
• So, $\text{splitting}\text{energy}{\text{Δ}}_{\text{o}}$ will be small.
• As a consequence electrons will easily get transmitted to the higher energy level due to the small energy gap in between. So, remains unpaired by occupying the higher energy level.
• Thus, results in high-spin complexes.
• The wavelength of absorption corresponding to the small $\text{splitting}\text{energy}{\text{Δ}}_{\text{o}}$ will be of smaller wavelength.
• Hence, the wavelength of absorption has a high frequency and higher energy.

So, if the colour of a given complex is of larger wavelength, then the complex is said to be a high-spin complex with the weak field ligand, since the larger wavelength gives the smallest splitting due to its smaller energy.

In contrast, if the colour of a given complex is of smaller wavelength, then the complex is said to be a low-spin complex with the strong field ligand, since the smaller wavelength gives the largest splitting due to its greater energy.