28 Jul 2008

Electron Counting and a Catalytic Reaction

In-Class Activity

Submitted by Patrick Holland, Yale University
Categories
Prerequisites: 
Corequisites: 
Course Level: 
Topics Covered: 
Subdiscipline: 
Description: 
This Challenge shows a catalytic process, and especially gives practice on electron-counting in transition metal complexes. Inorganic Challenges are exercises designed to be solved by a small group of students. Some Challenges practice a problem-solving algorithm, some reinforce important concepts, and some involve creativity or games. You can pick and choose Challenges from our Web site to increase active learning in your classroom, and we ask that you contribute creative Challenges of your own to give a head start to teachers at other colleges and universities!Please visithttp://chem.rochester.edu/~plhgrp/iicf/subjects.htm
AttachmentSize
Microsoft Office document icon 6 electron counting.doc33.5 KB
Learning Goals: 
(1) Students should be able to determine the total electron count of complexes and the oxidation state of the central metal ion. (2) Students should generalize the exception to the "18-electron rule" for square-planar d8 metal complexes. (3) Students should understand the terms "oxidative addition" and "reductive elimination." (4) Students should recognize the cyclic nature of a catalytic process.
Equipment needs: 
none
Implementation Notes: 
This is designed to be assigned to small groups of 3-4 students, who work through the steps of the Inorganic Challenge.
Time Required: 
1 hour
Evaluation
Evaluation Methods: 
No formal assessment of the benefits of this exercise has been conducted. The benefits of this exercise could be ascertained by asking a related question on a subsequent exam.
Evaluation Results: 
Qualitative monitoring of the students during the exercise indicates that students do not internalize electron counting and oxidation state calculation after lectures only. Common errors include: (a) counting neutral ligands as anionic, (b) forgetting to account for overall charge on the complex in oxidation state assignment, (c) mixing ionic and neutral electron-counting schemes. These problems can be corrected as part of group discussion. Because this catalytic cycle has complexes with different oxidation states, ligand types, and coordination numbers, they encounter a range of situations for identification of misconceptions.
Creative Commons License: 
Creative Commons Licence

Comments

If someone has tried this and has some student learning assessment results, could you please post them as a comment? Thanks!

I used this as an in-class activity last fall to introduce organometallic reactions and catalysis but, of course, have no formal assessment results.  I thought it worked for my purpose.  It helped illustrate why electron counting is helpful.

I used a modified version (attached) of this as a problem set in my upper level inorganic course toward the end of the semester, after we had covered organometallic reaction classes, valence electron counting, and trans effect. My modifications changed it from a guided exploration of the topics to an assessment of student comprehension of the concepts (i.e. - could they go from oxidation states and valence electron counts to the type of reaction that was occurring, could they determine preferred geometries based on d electron counts, etc.). This assignment could easily be adapted for use with another catalytic cycle, either for a variation on this problem set or for a quiz or exam.

Students were allowed to work in groups and had a week to complete the assignment. They generally did very well (class average 26/30 points), but this may be because we had worked through similar examples in class. I also included similar questions, with different catalytic cycles, on an in-class exam and the final exam, which they did very well on.

I also used the attached worksheet as an in-class activity to introduce organometallic reactions and catalysis this semester in an advanced inorganic chemistry course.  Students worked in groups of three or four to complete the activity.  I thought the activity helped reinforce the importance of electron counting and pi-bonding effects in coordination complexes, which we had previously covered, as well as introduce organometallic catalysis.  Unfortunately, I was not able to collect formal assessment results from this activity this semester.

I have also given essentially the same question on my final exam (I give the same catalytic cycle and ask students to do the electron counting for all of the metal complexes).  I have also covered the basic organometallic reaction types and ask them to give a name to each type of reaction).  I think it is a great question.  Students do fine with this question on the exam.

 

This is a classic reaction and serves as an excellent in-class activity. The reaction arrows in the middle of the cycle between water and HI need to be reversed for better understanding.

I have a modified picture if someone prefers to use it.

https://drive.google.com/file/d/0B56jc5AxtXDZUjZPTnN3MnQzUlE/view?usp=sh...

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