Submitted by Adam Johnson / Harvey Mudd College on Sat, 06/13/2020 - 19:30
My Notes
Description

I have long taught CFAE as a topic in my inorganic class, but only over the last 3-5 years has the concept really solidified with me. I think this exercise really does a good job explaining that even a simple theory can have predictive power. I have not seen this analysis in a textbook. Lots of books discuss CFAE and its impact on rate, but taking it to the next step and benchmarking it on real data and ∆o values, I have not seen. Maybe it is because I am not a kineticist and don’t see the “obvious” implications of this. In any event, I wanted to track down some real data to benchmark the claims and I think that this worksheet does a nice job summarizing the theory, and why it works, and with real data to support it.

Note to instructors, I do not differentiate in my class between CF and LF concepts. I only discuss CF and then move on to MO theory.

Attachment Size
CFAE LO in word format 30.68 KB
CFAE LO in pdf format 151.02 KB
Learning Goals

students will calculate crystal field activation parameters from relative orbital energies

students will see the concept of crystal field activation energy

students will use his/her knowledge of CFAE to calculate relative rates for ligand substitution reactions

Students will understand the limitations of a simple theory

Equipment needs

none

 

Implementation Notes

I have only done this as a lecture, specifically comparing Mn(III) and Cr(III) water exchange rates using ∆CFSE concepts. However, I finally tracked down some good representative data for both hs and ls complexes and I hope to use this worksheet in class or as a homework assignment in the future.

I hope to include a few slides about this topic to help illustrate some of the concepts of activation energy at a later date. To introduce it in lecture class, I draw a reaction coordinate free energy diagram with an octahedral complex as the starting complex and a square pyramidal complex as the transition state. I then estimate the activation energy as "one ligand bond strength" and the additional activation energy as the ∆CFSE.

An instructor may wish to have the students derive the ∆∆G equation instead of just giving it to them, in which case, simple modifications to the document would allow this to be another evaluation of student understanding. I suppose it depends on how heavily you teach kinetics before this point. I do not teach a lot of kinetics so for my class, giving them the equations is definitely the right call... for now.

Time Required
30-50 minutes

Evaluation

Evaluation Methods

I would grade the calculated values as correct or incorrect. The more open ended responses would probalby not be graded but I might take the general sense of the class and lead a discussion on misconceptions or important points in the next class period.

Evaluation Results

TBD!

Creative Commons License
Attribution, Non-Commercial, Share Alike CC BY-NC-SA