What is the no crossing rule in a Tanabe Sugano diagram?

The no crossing rule in a Tanabe Sugano diagram states that weak field terms must bend apart. This rule is rooted in the interaction between electronic states in transition metal complexes as affected by ligand field strength.

In these diagrams, the energy levels of different electronic terms are plotted against increasing ligand field strength. The no crossing rule reflects the physical principle that the energy levels of different multiplets (terms) associated with specific spin states will not cross as the ligand field strength changes. Instead, the term with higher multiplicity (e.g., a triplet state) will generally move farther up in energy and bend away from lower multiplicity terms (e.g., a singlet state), as the splitting between their respective energy states adjusts according to the strength of the ligand field.

This phenomenon helps chemists predict and understand the electronic transitions and spectra of transition metal complexes in various ligand environments. Understanding this rule is crucial for interpreting absorption spectra and theorizing the behavior of coordination compounds in different ligand fields.