7 Feb 2012

Gallium Chemistry: To be or not to be a Triple Bond!

In-Class Activity

Submitted by Sibrina Collins, The Charles H. Wright of Museum of African American History
Categories
Description: 

This learning object focuses on concepts of metal-metal bonding. In the late 1990’s, Greg Robinson published two research articles focused on the first gallyne, containing a Ga-Ga triple bond, and the first ferrogallyne, which contained an Fe-Ga triple bond.  I use an article published in Chemical and Engineering News (Dagani, R. “Gallium ‘Triple Bonds’ Under Fire” Chem. Eng. News, 1998, 76(11), p31-35) to introduce students to the controversy about the ferrogallyne and gallyne compounds and several “rebuttals” provided by Philip Power and the late F. Albert Cotton.  This activity consists of two components, namely presenting the ideas on metal-metal bonding by Robinson, Cotton, and Power and writing an article critique.

AttachmentSize
Microsoft Office document icon InClassMetalMetalBonds.doc31.5 KB
Learning Goals: 

There are various goals for this assignment, which reinforces concepts the student has learned in earlier courses.

 

1) The student will apply concepts focused on atomic orbitals.

2) The student will apply concepts about orbital interactions.

3) The student will apply concepts focused on various bonding theories (e.g. molecular orbital theory).

4) The student will re-evaluate the concept of both triple and double bonds and apply this idea to heavy main-group elements.

5) The student will write an article critique focused on Robinson, Cotton, or Power’s publications.

Equipment needs: 

Chalk board and/or Powerpoint Point

Implementation Notes: 

All students are required to read the 1998 Chemical Engineering News article describing the “controversy” surrounding Robinson’s discovery and three additional articles published by Robinson (Organometallics 1997, 16, 4511-4513), Cotton (Organometallics 1998, 17, 128-130), and Power (JACS, 1998, 120, 2202-2203).  A group of six students then were selected to present this topic to the class. Within the group, a team of two presented the arguments of Robinson (triple bond), a second team presents the arguments of Cotton (single bond) , and a third team defends the argument of Power (double bond). The students are given “loose” guidelines on presenting the topic to the class, specifically they should present a powerpoint or chalk talk, and they need to engage the audience.

Time Required: 
One class period for the in-class activity
Evaluation
Evaluation Methods: 

The students not presenting to the class provide feedback to the focusing on what worked well and areas for improvement.  The instructors of the class provided feedback focusing on various areas including, level of preparedness, level of knowledge demonstrated, balanced contributions from team members, and level of engagement with the audience.

Evaluation Results: 

Overall, the students that presented this topic to the class did an excellent job.  First, they engaged the students by distributing a handout to work in teams and review concepts such as bond order, how bond length is related to bond order, triple bonds, double bonds, and molecular orbital theory. After a brief discussion on these ideas, the students gave an over powerpoint presentation focused on the development of bonding theories (Lewis, Heitler-London, Pauling, and MO theory). Next, each team vigorously defended the results of each researcher.  At the end of the class period, most students in the class concluded that Robinsons’ compounds don’t contain a triple bond, but lies somewhere between a single and a double bond.  The students also pointed out that there was a “lot of pride on the line,” because at the time, no one would back away from their original position.  During the discussion, I wanted the students to explore concepts of orbital interacts.  More importantly, I wanted the students to realize that they cannot use "traditional concepts" that relate bond length to bond order in order to describe the bonding interactions between the heavier elements.

Creative Commons License: 
Creative Commons Licence

Comments

I used Sibrina’s learning object in my Advanced Inorganic Chemistry course in Spring 2012, following class discussions of metal-metal bonds and the MO descriptions of these in classic examples such as [Re2Cl8]2-.  I assigned The Gallyne Debate while I was away at the Spring ACS meeting in San Diego.  Students had one week to read all 4 papers, organize themselves into teams (there were 10 students in the class), and then present their findings and lead the class discussion on the day after I returned from the meeting.  I used Sibrina’s handout, with attribution, of course, with some additional wording about organization and evaluation.  Here is what I added:

Divide into 3 teams to present and lead a class discussion of this controversy: One team should present and defend Robinson’s position, one team should present and defend Cotton’s position, and one team should present and defend Power’s position.  Your presentations could include powerpoints or a “chalk talk” at the board, whatever your fellow team members think is most effective.

Your work on this project will be evaluated by me and by your peers on the basis of: your level of understanding, your team’s preparation and presentation, your engagement in class discussion, and your contributions to the team effort.

One issue that cropped up was which paper to assign for Powers’ double bond position.  I used the reference that Sibrina provides on the handout (J. Am. Chem. Soc. 1997, 119, 11705-11706.), but this is a different paper than the one linked in the Web Resources field above (J. Am. Chem. Soc. 1998, 120, 2202-2203.).  I do not know which Powers paper is more appropriate, or perhaps a different one altogether.  I will say that my students had a very tough time understanding how the former paper, the one I assigned, tied into this debate.  And I was not around to help figure this out.  So, I would love to hear some insight from other VIPEr users as to how to help students with this one.

I am attaching below the Peer Evaluation Form that I passed out and asked students to turn in after the class was over. 

In general, this activity was great!  I think the students put forth a good effort and learned a lot.  At one point during our discussion, one student said, “I feel like we are spending all this time talking about What is a bond?”  I replied that it did not get any better than that as far as I was concerned! 

The students repeatedly asked for more structure and clarity up front from me regarding expectations for the assignment and what should be in their presentations.  I found that interesting that they wanted more direction (these were juniors and seniors) rather than less.  At the beginning of the class, they asked me for example, which order they should present in, and I said that was up to them (clearly they had not thought about this).  In the evaluation forms, the students also asked for more advance preparation with me to help understand the arguments for a particular position – the double bond position was toughest, to understand unfamiliar topics from the papers such as NBO analysis, or to double-check MO diagrams and electron counting before they were presented to the class as a whole.  If I do this again, I might also read some more myself and ask students to look into where does the debate stand some 15 years later?

Maggie,

These are great comments. I believe I selected Powers JACS 1997 paper because that is where he first discused the Ga-Ga triple bond work from Robinson. Powers 1997 paper focuses on multiple bonding in Sn-Sn compounds. I am not sure which paper is the best to use. I am thinking about ways to expand this study. Perhaps I could have the students do some DFT calculations to see if they support a triple bond. But, Powers emailed me awhile back stating that he thinks Cotton actually was the first to suggest a double bond after vehemently defending his single bond position. I think Cotton concluded this based on DFT data.

Sibrina

 

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