Question

Suppose that you want to determine whether a ligand X is a weak-field or strong-field ligand. Would you synthesize an octahedral complex of ligand X with Cr3+ or Co3+? Explain your reasoning and illustrate with relevant appropriate crystal-field energy diagrams for each complex.

Answer 1

In order to be able to determine whether a ligand is weak field or strong field, we should prepare a complex such that after the measurement of a particular property of that complex we should be able to tell whether that complex has a strong ligand or weak ligand.

One such property is the Magnetic Moment of complexes.

Complexes with unpaired electron are paramagnetic and have a net magnetic moment.

Complex with no unpaired electrons are diamagnetic.

For CrX6 complex with Cr3+

Cr3+ has 3-d electrons.

Here we observe that both the strong and weak ligand octahedral complex have the same splitting and thus result in complexes that can not be distinguished by magnetic moment as both have 3 unpaired electron and are paramagnetic.

Now coming to Co3+

It has 6 d electrons.

Here, the weak field complex has 4 unpaired electron and is paramagnetic, whereas the strong field complex has no unpaired electron, and hence is diamagnetic.

Because we can differentiate the weak field complex and strong field complex of Co3+ using a simple magnetic moment test, we will choose Co3+ as a way to identify whether the ligand is strong field or weak field.

If the ligand is weak field, the complex will be paramagnetic.

If the ligand is strong field, the complex will be diamagnetic.