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