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Over the past several years, I've been doing this in-class exercise shortly after discussing mechanisms of ligand exchange. The exercise expands on the lecture material by having the students think about metal ions, rather than ligands, exchanging from a coordination complex. The students are encouraged to work in groups of 3-5 and actively discuss the material amongst themselves before we go over it as a class. I do not provide the students with the article ahead of time, so that they may come up with their own conclusions, as opposed to simply repeating those of the authors. I do encourage the class to read the article afterwards.
One item that I stress to my students is that coming up with a working mechanism is not usually a straightforward process. In certain cases, one needs to carefully weight contradicting bits of information, as the authors did in the source manuscript. Further, whichever mechanism one eventually settles on remains, at best, an educated guess.
In the current study, the rate law is inconsistent with an associative-type mechanism in which the iron reacts with the bimetallic complex before the loss of the sodium. This would seemingly be inconsistent with the negative entropy of activation. This parameter, however, will be influenced by solvation, and a more ordered outer-sphere in the transition state would explain this value and obscure a dissociative-type process. It may very well be that the sodium coordinates to water molecules before its dissociates from the ligand.
The exercise is meant for an upper-level class since it requires a solid understanding of kinetic parameters and kinetics. I have presented the exercise after introducing A, D, and I mechanisms for ligand exchange and discussing several case studies involving square planar and octahedral coordination compounds. I typically give the students about 10-15 min to work on this.
| Attachment | Size |
|---|---|
| Powerpoint with in-class exercise | 68.11 KB |
The goal would be to get the students to think more critically about the data that one can acquire in a mechanistic study. In the course of the exercise they will need to evaluate how reliable each datum is and put forward a mechanism that is consistent with the entire set of data.
None. This can be done on the board, although a projector would allow the crystal structure of the bimetallic compound to be presented more quickly and clearly.
Evaluation
I consider the exercise to be a success when there is a high level of discussion. The groups that more animatedly discuss the material usually come up with a more informed answer.
Periodically, some groups will just sit there and wait for other groups to present their thoughts on the exercise. Sometimes, they can be prompted to be more proactive. Other times...
Typically, the students struggle to weight the two seemingly contradictory bits of information. They know that first-order rate laws are associated with dissociative mechanisms and that negative entropies of activation are associated with associative mechanisms. Many of the examples that they previously encountered had everything agree and didn't require them to evaluate the limitations of the provided data. They are rarely asked to choose.
Many students will realize that the entropy of activation is an imperfect indicator and determine that the rate-determing step involves the loss of the sodium. About one third to one half, however, will not. These students will tend not to settle on a mechanism.
This said, most students have really liked this exercise as it does give them an opportunity to think more critically about the issues discussed in class.