10 Jan 2009

The 18 Electron Guideline: A Primer

In-Class Activity

Submitted by Nancy Scott Burke Williams, Scripps College, Pitzer College, Claremont McKenna College
Categories
Description: 

This consists of two parts (and a solution, which is linked below under "Related Resources", but for which you will need a faculty privileges): a primer for students (best if handed out prior to class so that students can read it beforehand, or delivered in pre-lecture format) and a worksheet. The worksheet is designed to be done in small groups with assistance from an instructor. In very large classes, in which the instructor cannot circle amongst the groups, the instructor can work through each example after the groups have a few minutes to work the problem on their own. 

The idea is for the students to learn to "count to 18" by figuring out their own rules, given that the compounds are 18 (or 16) electron compounds rather than telling them the rules for various ligands. 

Omitted from this worksheet are the more complex cases: compounds with metal-ligand multiple bonds and nitrosyls, since these are often handled later in the organometallics unit by many instructors and since instructors and textbooks differ in their treatment of these ligands.

Learning Goals: 

Students will be able to determine dn electon counts, oxidation states, and overall electron count for most organometallic complexes.

Students will be able to explain the rationale behind the counting schemes and to articulate the pros and cons of each method.

Students will be able to identify and become familiar with a number of common ligands without time being specifically dedicated to a survey of these ligands.

Equipment needs: 

Students. Implements of writing.

Implementation Notes: 

See above.

Time Required: 
ca. 50 minutes (or more as minor homework)
Evaluation
Evaluation Methods: 

I primarily evaluate this by circling the room and watching them grow in ability to assign electron counts and justify their methods in their own words. They get pretty good at it by the end. In addition, I later progress to talking about catalytic cycles, and ask them to do electron counts for each species in several catalytic cycles. I can tell how well they've internalized these skills by their ability to approach these new problems.

Evaluation Results: 

As long as groups are chosen by the instructor (such that there is a range of ability in each group), most students are able to work through the problems successfully. Students seem to have the most difficulty with the very beginning, in which they have to do their "first electron count" and oxidation state/d-electron configuration count. 

However, most groups need some assistance at some point in the worksheet (some at several places), so it really is best to either constantly "check in"  and work through the problem on the screen (best for large classes) or provide helpful hints and let each group work at its own pace (for small classes).

If the students are unable to complete the worksheet in time, it can be completed for homework.

Creative Commons License: 
Creative Commons Licence

Comments

This looks great, Scott! I really like the way the worksheet guides the students, with a short explanation about the complex and a table that leads them through the counting. And I like the fact that the students have to count electrons "both ways." I will definitely use this next time I teach inorganic!

I used this in my class this week, and it was a very effective way to teach the students how to count to 18.  I thought the exercise would be trivial, but most of the students had good questions that we worked through.  They admitted that counting to 18 can be tricky after all!  The class seemed to split pretty evenly over who preferred which method (ionic vs. covalent), which I also found interesting.  One student commented that it probably depended on whether you were more familiar with common metal oxidation states or the organic fragments. The range of examples Scott uses on his worksheet is interesting and offers a nice introduction into a number of different topics.  For example, we went right on to talk more about dihydrogen complexes.  I definitely will use this again!

I used this in Inorganic II this semester.   It saved me when I was mixing up the two methods myself. 

 

Sheila SmithAssociate Professor of ChemistryUniversity of Michigan- Dearborn313-583-6399(office)734-788-8144 (cell)  

Glad to help out!

I used this activity in my junior/senior level class as well. What I really liked about it is that it builds up the skills gradually and by the end students felt pretty confident figuring out new situations.  I did answer quite a few questions as they went along, but by the end most (though not all) had the correct answers.   

I teach this method almost exactly the same exact way you do.  I have a similar handout which I will post and link back to here.  Generally, most students work through the examples I provide and don't have any problems.  The hardest part I have found is for students to recognize which ligands are X's and which are L's.

Adam

I have used this as a handout for when I had to be gone to a conference. I used the "Shark Tank" method - no introduction other than "here's your assignment for when I'm gone". It was very successful. I had to help them with a few of the concepts, but overall a great assignment.

Can anyone (Nancy) explain why [Fe(CO)4]- is drawn in what appears to be a see-saw geometry on question 1 on the worksheet?  I have used this worksheet before, but today I noticed that the geometry was significantly different from the tetrahedral geometry that I expected.  I thought perhaps this was an organometallic thing that I just didn't know, so I looked and found this paper: http://dx.doi.org/10.1021/ja00446a022. The widest C-Fe-C angle they mention is 154.7° when Cd2+ is the counterion, but the angles are close to 109° when the counterion is K+ or Na(crypt)+.  Is there a story here? Or is this really just chemdraw?

Above notwithstanding, I also like this worksheet because it does a good job helping students learn to identify L and X ligands for themselves rather than memorizing a bunch of rules.  In their preclass responses (JiTT) for this section, my students cry out for practice, and this worksheet answers the call.

I may be the first person ever to get through the entire primer, but today my students noticed that the Ni allyl dimer (or dimmer?) in the last problem, has the formula of allyl given as C3H3-.   Double checked with Nancy and this is, indeed, an error.  (What?!! I'm a bioinorganic chemist, what do I know?)

I made a pencast to correct the issues that we had in class.  Happy to share, but that doesn't seem to be allowed on a comment

 

I used some of the practice problems today as well as Adam's one-page intro.  It worked well - a good progression of difficulty.  Thanks, Nancy and Adam!

We totally need the CBE version of this. Maybe the next time someone teaches electron counting, they could assign students to do that! In the mean time, there are some great CBE resources already on VIPEr.

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