Question

Would it be better to use octahedral ${\mathbf{\text{Ni}}}^{\mathbf{2}\mathbf{+}}$complexes or octahedral${\mathbf{\text{Cr}}}^{\mathbf{2}\mathbf{+}}$ complexes to determine whether a ligand is a strong-field or a weak-field ligand by determining experimentally the number of unpaired electrons? How else could the relative ligand field strengths be determined?

Short answer

It would be better to use octahedral ${\text{Cr}}^{2+}$complexes to determine whether a ligand is a strong field or a weak-filed ligand. The relative ligand field strength can be determined by observing their colors in aqueous solutions.

Step-by-step solution

Why octahedral Cr2+complexes are better in determining the strength of ligands.

To solve this, we need to first write down the electronic configurations of each ion.

In the case of ${\text{Cr}}^{2+}$, there are four unpaired electrons and one empty orbital. Therefore with a weak-field ligand, the fourth electron will be jumped to the, e.g., orbital, whereas with a strong-filed ligand, the fourth electron will be paired.

Why Ni2+ cannot determine the field strengths of ligands.

As we can see from the figures that in this case of ${\text{Ni}}^{2+}$, only pairing can be done as there no empty orbitals are left. Therefore ${\text{Ni}}^{2+}$cannot determine the field strength of ligands.

How colors can determine the field strength of ligands.

We know that by observing the colors exhibited by complexes in aqueous solutions, we can find out the colors absorbed by them. And by the color absorbed by them, we can find out the wavelength absorbed and hence the CFSE, thus the ligand field strength. So, e.g., if a complex exhibits red color we understand it absorbs green color, which is a color of shorter wavelength. And by the formula $\text{E=}\frac{hc}{\lambda }$, we know colors of shorter wavelengths absorb more energy. From this, we can conclude that the incoming ligand must be a strong-field ligand.