What type of transitions are forbidden on the Tanabe-Sugano diagram?

In the context of the Tanabe-Sugano diagram, which is used to analyze the electronic transitions in d-transition metal complexes, spin state transitions are considered forbidden due to the spin selection rule. This rule states that electronic transitions involving a change in the total spin multiplicity are not allowed, meaning that transitions that involve a change in the spin state (for instance, from a doublet state to a quartet state) do not have a significant probability of occurring.

This is crucial in understanding how electronic configurations and their associated energies manifest in terms of absorption spectra. Specifically, if a transition requires a change in spin state, it generally does not result in a strong absorption band, as these transitions would be "spin forbidden."

In contrast, transitions that do not involve changes in spin states, such as those that remain within the same multiplicity but may involve changes in orbital angular momentum or electronic configuration, are allowed and more likely to be observed in practice. Thus, recognizing that changes in spin state are forbidden transitions on the Tanabe-Sugano diagram helps to explain the electronic behavior of transition metal complexes in various environments.