8 Mar 2009

Using Computational Chemistry to discuss backbonding to CO

In-Class Activity

Submitted by Lori Watson, Earlham College
Categories
Description: 

This activity uses Gaussian with the WebMO interface to investigate the role of the metal in backbonding to CO as well as effects of the trans ligands. It can also be used as a way of introducing computational chemistry in an inorganic course.

Learning Goals: 

Learning Goals: After this exercise you will be able to…
•    Explain the effect of changing the metal on the CO stretching frequencies of a metal carbonyl complex
•    Understand the role of σ and π donors/acceptors in modifying the amount of π backdonation into a trans CO ligand
•    Predict the degree of backdonation into a trans carbonyl that will be observed with an unfamiliar ligand
•    Perform DFT calculations to find the minima and vibrational frequencies of a molecule using WebMO/Gaussian.

Equipment needs: 

You will need some kind of computational program, preferably one that does DFT calculations. Gaussian with the WebMO interface is suggested.

Implementation Notes: 

I have attached some of the Gaussian output files of the complexes. They can be imported into WebMO and used as starting geometries. If you do not have your own computational program, they can also be imported into the WebMO Demo site (http://www.webmo.net/demo/index.html) and students can look at the outputs and animate the vibrations, etc.

Time Required: 
If the students have never used WebMO/Gaussian before, 1-2 hours. If they have, it can be assigned as homework. The calculations (depending on how good their guesses are) take betwen 15minutes and 3 hours to complete.
Evaluation
Evaluation Methods: 

Student learning is primarily assessed by means of a written report describing their calculations and results. Related concepts of pi-backbonding, pi donor/acceptor, trans effect, etc. are the subject of problem set and exam questions.

Evaluation Results: 

Typically they do well with this, and enjoy calculating "real" molecules--once they get the hang of the software. Some students do not enjoy learning new software programs and find drawing the molecules and editing the z-matrices tedious and difficult. Students universally like to see the vibrational animations! I am careful to discuss "error bars" of computational calculations--especially the qualitative vs. exact quantitative CO frequencies--as students will otherwise assume the computational results are more accurate than the experimental ones.

Creative Commons License: 
Creative Commons Licence

Comments

Lori,

I hope all is well. We don't use WebMO, but we do use GaussView. Any thoughts? I would like to use this in my Advanced Inorganic course as a homework assignment.

Sibrina

Sibrina,

GaussView will do everything WebMO does for a local installation of Gaussian (at least in my experience).

Adam

Dear Lori,

We've accepted the great backbonding challenge and have run into two questions.

1. Is it okay if the molecule distorts from octahedral? For one of our molecules, the L ligand really wants to "lean" in one direction.

2. How "pure" does the trans-CO stretch have to be? When we look at the stretches in the CO region, there is usually one that is mostly trans-CO, but it often has some stretching of the other ligands going on.

Thanks!

Joanne,

1.  I'd say "Yes, but..." You might ask yourself why it's leaning.  For example, you might image a complex where it is distorting because there is hydrogen bonding between ligands going on.  This may lengthen the M-L bond "artificially" and so make it not as electron donating, etc. as it might otherwise be.  You will also sometimes get distortions if you choose an oxidation state of the metal that makes it unlikely for it to "want" to be 6 coordinate.  You might have one ligand then really weakly bonding, which will of course also then effect your electron donation for reasons other that what you're trying to have students investigate.

 

2.  Well, this is a tough one!  For the purposes of this exercise I just have students pick the one that is MOSTLY the trans CO stretch.  But yes, small amounts of other stretching will also sometimes occur.  There probably is a group theory explanation??

 

Hope this helps!

I've updated the student handout for this exercise and will attach it as soon as I figure out how to. (In the mean time, feel free to contact me if you would like it.) We are currently using WebMO 17. The students really like this exercise, although they have to be warned to start early because some of their calculations take a long time. They also struggle with getting the multiplicity right (as do I) when they have anionic complexes.

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