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This activity works to reinforce the various factors that affect M --> CO backbonding and the resultant effects on CO bond stretch. The attached properties are written on notecards and the students broken up into small groups (3-4 per group). They are given a selection of the cards (making sure the same group doesn’t get both items of a pair) and poster putty and asked to divide the cards up into factors that raise the CO stretching frequency and factors that lower it. Those two categories are written on the board and the students then pin up the cards in their appropriate spot. After all of the groups are done, I indicate whether everything is correct (or not). If some cards are in the wrong spot, the students have to come to consensus about which cards have to move.
| Attachment | Size |
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| IR backdonation list.docx | 75.23 KB |
A student should be able to apply his/her knowledge of factors influencing the degree of backdonation into the pi* orbital of CO to predict (qualitatively) CO bond stretch frequencies. The student should understand related factors (i.e. bond length, bond order) and how they are related to the CO stretching frequency.
Note cards and poster putty and a chalk/white board. One could also use post it notes instead.
I used this in my Organometallics course (10 students) in the spring of 2012, but could also imagine using it in my regular Inorganic course. The students had read some background reading on backdonation for class, and about half of them had already had Inorganic Chemistry in which we talk about backdonation and its effects briefly. I split the students up into three groups and gave them 5 minutes to put their cards into the two categories on the board. They initially got several incorrect. In most cases it was obvious as both cards of a pair (for example: increase in pi donation/pi acidity of the trans ligand) ended up in the same category. The students discussed the incorrect cards, came to a consensus about which ones to more, and moved them. All were correct by the 3rd try. One could expand the number of cards (which would be necessary for a larger class) by including a number of cards with pairs of compounds on them (i.e. Ti(CO)6−2 , Fe(CO)62+) and ask whether the 2nd compound has a higher or lower IR stretch than the former.
Evaluation
On the next lecture day, I gave a quick ungraded “quiz” with many of the factors listed and asked the students to circle those that would raise the IR stretching frequency. On the next formal quiz after this learning object I had a question with three pairs of compounds and asked them to circle which had the lowest IR stretching frequency and explain why. I gave a similar question on the midterm exam.
10/10 students were able to circle the correct factors that would raise the IR stretching frequency on the day after the activity. On the formal quiz, the three part question was worth 9 points. The average number of points awarded was 7.3/9, with a high of 9 and a low of 2.5. The most common mistake was on determining what would happen with backbonding to CO when considering trans ligands of different types. One student had the reasoning correct but reversed which way the IR stretch went when backbonding increased. On the midterm exam on the 3 point question most directly assessing their understanding of factors that influence back donation, the average score was 2.8.