27 Jun 2013

Molecular Structure - The Curious Case of Iron Tetracarbonyl

In-Class Activity

Submitted by Zachary Tonzetich, University of Texas at San Antonio
Categories
Description: 

This in-class activity traces the many contributions leading to the correct assignment for the solid-state structure of triiron dodecacarbonyl, [Fe3(CO)12],  with the aim of reinforcing ideas about IR spectroscopy and group theory. I give this activity to my advanced inorganic chemistry class (graduate students and senior undergrads). The activity is loosely based on the paper: Desiderato, R., Jr.; Dobson, G. R. J. Chem. Educ. 1982, 59, 752-756 and incorporates questions about symmetry and group theory for metal carbonyls. The idea is to have students think critically about experimental data (some of which is provided) and to have them consider how their predictions from group theory match up with historical conclusions drawn by the original researchers.

AttachmentSize
PDF icon Iron tetracarbonyl.pdf1.72 MB
Learning Goals: 

A student should be able to apply his or her knowledge of symmetry, group theory, and selection rules to answer each of the questions described in the activity. Furthermore, the students should develop an appreciation for the process of structure assignment based on spectroscopy and hopefully a little bit about the history of inorganic chemistry.

Equipment needs: 

None.

Implementation Notes: 

Having the original experimental papers handy is nice to quote the original authors' conclusions.

Time Required: 
75 minute lecture period
Evaluation
Evaluation Methods: 

This exercise is usually used prior to exams to help review concepts in IR spectroscopy and group theory. I judge the students performance on how well they can answer the indicated questions, and also by the flow of the discussion (do they pick up on errors made in the original conclusions reached in the referenced papers). I then use similar questions concerning point group assignment and IR-allowed normal modes on exams.

Evaluation Results: 

Students typically do quite well and are able to assign the point groups correctly after some thought as well as assign symmetries to normal modes. As usual, converting the two-dimensional drawings on the screen to three-dimensional models in their heads can cause confusion. The questions concerning polarization are usually the most difficult but many of the students are able to answer them correctly.

Creative Commons License: 
Creative Commons Licence

Comments

I utilized this module in two parts during an upper level (master students + advanced undergraduates) inorganic course that focused on spectroscopic and hands-on computational methods (using ORCA). The first part was used when reviewing group theory, IR and Raman. We revisited the exercise when learning about Moessbauer spectroscopy later in the course. Some students initally struggled to remember group theory, but were able to work through the problems. The exercise was particularly successful and interesting for students because it contained examples from the literature and also addressed challenges in characterizing complex systems. Many students were particularly fascinated by the experimental difficulties and limitations presented in this module. For a course focused on correlating spectroscopic parameters with computational work, this worked very well without significant modification.

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