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In this paper (Llewellyn, Green and Cowley, Dalton Trans. 2006, 4164-4168) the synthesis and characterization of two cobalt compounds with an N-heterocyclic carbene ligand (IMes) are reported. the first, [Co(CO)3(IMes)Me] was prepared by the reaction of [Co(CO)3(PPh3)Me] with IMes. The second compound, [Co(CO)3(IMes)COMe] is formed by the addition of Co to the first. The CBC method for counting was employed to have the students determine the classification, electron count, and metal valence for the compounds. The structures of both compounds are reported and students are asked to examine the data to explore the bonding interactions in these molecules. In addition, the IR spectroscopy provides a good opportunity to use group theory to further examine the bonding in these molecules. Finally, the second compound is converted to a hydride by the addition of H2. The hydride was examined as a catalyst for hydroformylation. While this LO has 17 questions, it can certainly be adapted for use by removing some of them depending on the course. For example, the formation of the hydride and the catalysis can easily be omitted.
This LO was created in honor of Professor Malcolm Green who passed shortly before it's creation. Although I did not know Professor Green very well (we exchanged emails once and I saw him speak), his Covalent Bond Classification (CBC) method for counting has had a profound on my teaching and research. I was first introduced to organometallic chemistry as an undergrad. At that point I was taught the ionic method of counting. There was a lot about it that I liked, but I also recall discussions in grad school from people in the covalent counting school that left me questioning the ionic method. But I sure liked my iron being Fe(II) in ferrocene, so I stuck with the ionic method. Several years later I had Ged Parkin visit Lafayette and it was through that meeting that I drank the Kool-Aid. Somehow I fooled some folks into thinking I am an expert in this method. I continue to learn. But it works so well and my students seem to grasp it very quickly. I continue to work at bringing these ideas into my General Chemistry I course. So, although he sadly will never know it, Professor Green has had a huge impact on chemistry majors at Lafayette (and many other institutions).
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| Questions for students | 20.81 KB |
Students should be able to
Use the CBC method to count electrons in three closely related cobalt compounds
Examine X-ray structures (in particular bond lengths) and relate that data to bonding
Use group theory to predict the number of IR active CO vibrations
Analyze the results of a paper to make predictions about possible mechanisms
Suggest additional experiments to be performed to help explain results of the paper
This is probably too long to do in a single class period. Students should certainly be able to do the counting questions at home. The Group Theory would also be good to have completed before class. Their answers could then be reviewed quickly prior to moving on to the rest of the quesitons.
Evaluation
I haven't used this in a class yet. However, as this was created in the year of COVID-19, it is very likely I will use it as an exam in my fall 2020 course.