For years, I spent 2-3 days a semester working through Tanabe-Sugano diagrams, their development from terms, their evolution from Orgel diagrams, their analysis to give transition energies (the old ruler- trial and error analysis) and nephalauxetic parameters. Recently, colleagues in VIPEr convinced me that my time in class could be better spent, but I am not willing to completely give up on Tanabe-Sugano. For that reason, I have developed this exercise that boils the application of T-S diagrams down to an exercise in interpreting graphs (a skill which is sadly lacking in some of my students). It omits some of the analysis of the T-S diagram, but I think it gives adequate coverage to what is truly the most useful information to be gleaned from T-S diagrmams.
A student should be able to identify the appropriate T-S diagram to use for a particular metal complex. A student should be able to predict whether a complex will be high-spin or low-spin and relate this to a particular position on the T-S diagram (x-axis). A student should understand that electronic transitions will not affect the value of either DelO or B, therefore any transition should be represented by a vertical line. A student should be able to predict the spin allowed transitions for a transition metal complex based on the appropriate T-S diagram. Given the necessary information to determine an exact x value (DelO/B), a student should be able to predict the energy of a particular electronic transition. A student should be able to convert n energy in wavenumbers to a wavelength in nm.
It may be helpful to provide students with rulers / straight edges. Students will need calculators.
It is helpful to have available for students a complete set of T-S diagrams, either from your textbook or photocopied.
As an in-class exercise, this activity is evaluated based on the quality of the discussion that is generated. Students work together in groups to apply their knowledge of electronic structure and graphs to answer the questions posed. It can be easily modified to serve as an exam -question for more direct assessment. For an exam, I may skip the first question, providing students with a single T-S diagram, and asking questions about a complex based on that T-S diagram.
Usually ,this particular example spurs some debate about whether the [Mn(H2O)6]3+ complex is in fact high-spin (because of the 3+ oxidation state. This in itself is a valuable review since it requires student reasoning of the effects on spin of metal identity, oxidation state and ligand field strength.