11 Feb 2015

In-Class Review Questions for Metal Carbonyl Complexes

In-Class Activity

Submitted by Christian R. Goldsmith, Auburn University
Categories
Description: 

The slides provide review questions for a senior-level treatment of the spectroscopy and reactivity of metal carbonyl complexes. These are intended to be dispersed through one to three class periods.

The first slide is a review of electron counting and the 18-electron rule.

The second slide quizzes the students on the relationship between the electron-density of the metal center and the strength of the C-O bonds in the carbonyl ligands. It is intended to be given after a discussion of how IR can be used to assess the strength of M-C and C-O bonds in the compounds.

The third slide has students make predictions about the acidity of metal carbonyl hydride complexes. For the most part, this is a review of concepts that students would have encountered in general and organic chemistry. The last question brings in hard soft acid base theory and forces students to think about the nature of the M-H bond. I present this after a discussion of metal carbonyl reactivity, specifically their reduction to metal carbonylates and subsequent protonation to metal carbonyl hydrides.

The fourth slide can be used to link the spectroscopic characteristics of the metal carbonyl complex to its reactivity. I present these questions after a discussion of how the electron density of the metal center biases the compounds towards reactivity with electrophiles or nucleophiles.

AttachmentSize
File The class exercise130.71 KB
Learning Goals: 

The goal is to allow students to apply the knowledge that they have just learned about metal carbonyl complexes. The students will specifically correlate the spectroscopic characteristics of these compounds to their reactivity. One overarching theme that can be stressed throughout the lecture is that the electronic characters of the metal ion and ligands greatly influence not only the IR stretching frequencies of the carbonyl ligands but also their reactivities.

Equipment needs: 

Besides a projector and a computer, there are no special equipment needs. A blackboard or white board may facilitate discussion of the answers.

Implementation Notes: 

I typically have the students begin each exercise by working individually. Upon arriving at their independent answers, I have the students share them. First, they discuss their answers with one or two partners. Afterwards, the groups share their answers with the class. I will then supplement their answers by noting things that they may have missed.

Throughout the discussion period, I will walk amongst the students to gauge how they are doing with the exercise. If they appear lost or are on the completely wrong track, I'll offer a hint to help them out.

Time Required: 
5-15 min per slide
Evaluation
Evaluation Methods: 

Mostly, I am looking for a good discussion of the material. Smaller classes and ones with students who are not accustomed to group exercises may require more prompting to work together.

Certainly, students who provide more correct answers to the questions demonstrate that they understand the material to a higher degree.

Evaluation Results: 

I have found that most students handle the material very well. Most of their predictions (80-90%) are correct and require minimal input from myself.

With respect to the electron counting, some students have difficulty with the nitride ligand and the metal-metal bond.

Students have the most difficulty gauging the electron-donating/withdrawing abilities of the different phosphine and carbonyl ligands. They usually need to be told that Cp- ligands are electron donating relative to carbonyls. The students in my classes have had very little experience with Hammett relationships and do not have a good feel for what substituents are electron-donating and electron-withdrawing. Those with a third semester of organic chemistry would be anticipated to handle this portion more easily.

Students normally correlate the IR stretching frequencies of the carbonyl ligands to the preferred mode of reactivity without any problems.

Creative Commons License: 
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