Orgel diagram

Orgel diagrams are correlation diagrams which show the relative energies of electronic terms in transition metal complexes, much like Tanabe–Sugano diagrams. They are named after their creator, Leslie Orgel. Orgel diagrams are restricted to only show weak field (i.e. high spin) cases, and offer no information about strong field (low spin) cases. Because Orgel diagrams are qualitative, no energy calculations can be performed from these diagrams; also, Orgel diagrams only show the symmetry states of the highest spin multiplicity instead of all possible terms, unlike a Tanabe–Sugano diagram. Orgel diagrams will, however, show the number of spin allowed transitions, along with their respective symmetry designations. In an Orgel diagram, the parent term (P, D, or F) in the presence of no ligand field is located in the center of the diagram, with the terms due to that electronic configuration in a ligand field at each side. There are two Orgel diagrams, one for d¹, d⁴, d⁶, and d⁹ configurations and the other with d², d³, d⁷, and d⁸ configurations.

In an Orgel diagram lines with the same Russell – Saunders terms will diverge due to the non-crossing rule, but all other lines will be linear. Also, for the D Orgel diagram, the left side contains d¹ and d⁶ tetrahedral and d⁴ and d⁹ octahedral complexes. The right side contains d⁴ and d⁹ tetrahedral and d¹ and d⁶ octahedral complexes. For the F Orgel diagram, the left side contains d² and d⁷ tetrahedral and d³ and d⁸ octahedral complexes. The right side contains d³ and d⁸ tetrahedral and d² and high spin d⁷ octahedral complexes.

...
Wikipedia