This in-class activity explores the electronic structure and spectroscopy of the square-planar iron(II) sites in the mineral gillespite through a crystal field theory approach. This activity is designed for an advanced inorganic chemistry course where group theory and more advanced topics in ligand field theory are taught. The activity is based on the work detailed in the paper: Burn, R. G.; Clark, M. G.; Stone, A. J. Inorg. Chem. 1966, 5, 1268-1272. I typically use this activity to review for examinations because it brings together many of the central ideas I cover as part of crystal field theory such as orbital diagrams, term symbols, vibronic polarization, selection rules, and rudimentary magnetism.
The primary goal of this activity is to have students apply their knowledge of CFT and group theory to explain the electronic structure of a transition metal ion in a mineral (the original use of CFT!). The actiivty uses the example of a square-planar complex, which forces students to make a departure from their usual routine of evaluating octahedral and tetrahedral complexes with CFT. Students should also gain an appreciation for the technique of polarization in electronic spectroscopy and how it can be used to identify certain transitions.
I display the pdf file on screen in front of the class, being careful to keep the answers to any questions covered until the students have had time to work on them. Students work individually or in groups on each question and then we discuss each answer (including any questions students have) prior to proceeding through the document. The students will need a set of character tables. In addition, including correlation tables and direct product tables will expedite the activity (see web resources). Having the .cif file of gillespite handy is also nice to be able to show students using Mercury. I have attached one such file from the paper: Amer. Mineral. 1974, 59, 1166-1176 in the faculty files along with the MS word document of the activity in case people want to change any aspects.