9 May 2013

A DFT Study of Metal Pentacarbonyls

In-Class Activity

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

Metal carbonyls are the most widely studied organometallic complexes.  This exercise uses Gaussian with the GaussView interface to investigate the role of the metal centers on backbonding to the CO ligand. Density Functional Theory (DFT) methods were used to evaluate two classic metal pentacarbonyls, namely Fe(CO)5 and Ru(CO)5.

 

Learning Goals: 

There are various goals for this activity, which reinforces fundamental inorganic chemistry concepts learned in early courses.

  1. The student will apply concepts focused on symmetry and point groups.
  2. The student will apply concepts focused on bonding theories (e.g. Valence Shell Electron Pair Repulsion Theory).
  3. Perform DFT calculations to find the minima and vibrational frequencies using GaussView/Gaussian.
  4. The student will be able to compare metal carbonyl complexes and predict which should have the highest average CO stretching frequency.
Equipment needs: 

You need a computational program to be able to complete the DFT calculations. Gaussian with the GaussView interface is a reasonable method, but WebMO/Gaussian should work reasonably as well.

Implementation Notes: 

In addition, I distributed the handout “What is Computational Chemistry?” as an additional resource for the students. Prior to the beginning of the semester, I was able to obtain a classroom account from the Ohio Super Computer Center (OSC), which is housed on the campus of The Ohio State University.  Thus, I trained the students enrolled in my Advanced Inorganic course how to access the resources at the OSC. It was a small class with five students, so this worked out well.

Time Required: 
Building the molecules in the GaussView interface and submitting jobs to the OSC for the first time, took about an hour.
Evaluation
Evaluation Methods: 

After completing the calculations, the students were required to submit a technical report describing their method, results and discussion. The students needed to compare their theoretical predictions with the literature.

Evaluation Results: 

The students were evaluated on the quality of their reports. The challenge for the students really focused on getting comfortable with UNIX commands and building molecules in the GaussView interface. Eventually, the students were very comfortable with doing this. The students also enjoyed the vibrational animations. The geometry (TBP) average bond lengths and angles are consistent with the literature.

One key skill I wanted the students to learn in this course was to be able compare metal carbonyl complexes and predict which structure should have the highest carbonyl stretching frequency.  (Prof. George Stanley’s Organometallic Chemistry lecture notes were quite helpful with this discussion.) When evaluating complexes, they need to consider the nature of the ligands, electronegativity of the metal center, oxidation state, and the number of d electrons. Both the Fe(0) and Ru(0) are d8 atoms, but Fe is more electronegative (less electron-rich). Thus, we would expect the Fe atom to hold on to it’s d electrons and not engage in backbonding with CO as much when compared to the ruthenium system.  Now, the average CO stretching frequencies (gas-phase) for Fe(CO)5 were 2085.33 (equatorial) and 2136.35 (axial) cm-1. The average values for Ru(CO)5 were 2079. 69 and 2134. 95 cm-1. The literature reports an opposite trend (in hexane solutions) for Fe(CO)5, namely 2022.5 and 2000.5 cm-1. For Ru(CO)5 and 2036.5 and 2001.5 cm-1.  I have attached some results for the iron complex.

Creative Commons License: 
Creative Commons Licence

Comments

Sibrina,

This is a nice exercise.  I think it will help students link VSEPR theory with more sophisticated bonding models.

Anthony,

Thanks! This was my first time incorporating DFT in my Advanced course and it worked out well.

SNC

The VIPEr community supports respectful and voluntary sharing. Click here for a description of our default Creative Commons license.