Tanabe-Sugano diagram

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Tanabe-Sugano diagrams are used in coordination chemistry to predict absorptions in the UV and visible electromagnetic spectrum of coordination compounds.

A Tanabe-Sugano diagram is a graph which plots the energy of different spectroscopic terms (on the y-axis) against the size of the ligand field (on the x-axis). The units used for each are given in terms of the Racah parameter B (E/B for the y axis and Δ/B for the x-axis, where Δ is the ligand field splitting energy). This choice of unit means that the diagram takes account of electron-electron repulsion effects, which an Orgel diagram cannot. It is mainly used to predict the energy of any absorption bands that are predicted but not observed in the UV/visible spectra of transition metal complexes from the energies of those that can be seen, and can also be used to predict the size of the ligand field necessary to cause high-spin to low-spin transitions.

The lowest energy state is usually placed along the x-axis, (essentially as a baseline to an Orgel diagram and all other states plotted relative to it. If at different splitting energies different orbitals hold the lowest energy state (ie, the lowest lines on the Orgel diagram overlap), then the baseline stays with the lowest energy orbital, putting a kink in the diagram.) The number of curves intersected by a vertical line for a given Δ/B gives the number of possible transitions and therefore the number of expected spectroscopic absorption features. For more than 1 and less than 9 d electrons, there are more than 2 ways of arranging electrons in orbitals leading to different configurations, called Russell-Saunders (or spectroscopic) states.