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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}.

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CollinsComputationalChemistry2013.doc | 36 KB |

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

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

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.

#### Evaluation

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.

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 d^{8} 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.

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