Reaction of ligand substitution

Abstract

The purpose of this lab was to study ligand substitution in the preparation of Mo(phen)(PPh3)(CO)3 from Mo(CO)6, 1,10-phenanthroline, and triphenylphosphine. The intermediate product of Mo(phen)(CO)4 was analyzed using IR and UV-vis spectroscopy. The final product was then also analyzed using the same two spectroscopy methods in order to determine the affect of the ligand substitutions on the electronic and structural properties of the complex.

Introduction

It is possible to substitute a second ligand in for another ligand.

ML6+ L’=ML5L’+ L

This type of reaction is called ligand substitution. These occur only on metal centers though the mechanism of how varies according to the ligands under consideration1. Ligand substitutions can greatly affect the electronic and structural characteristics of different octahedral complexes4. There are three types of ligands. The first type of ligand possible is a ? donor ligand. These ligands are classified by having a single pair of electrons which are donated to the metal and have no double or triple bonds. In ?-donor ligand interactions with metals, there are no ? interactions. The second type of ligands is the ? donor. Classification of these ligands is their having more than one electron pair and no double or triple bonds. The third type of ligand is the ? acceptor. These ligands can be identified by their double and triple bonds or empty d orbitals. A ? interaction is present in both the ? acceptor and ? donor ligand to metal interactions2.

When ligands are trans from one another, there is an electronic effect on one ligand from the other, called the trans effect. The classical trans effect is when a stronger ? donor ligand causes the weaker ? donor ligand bond to weaken. This makes it easier for the weaker ligand to dissociate and react in a ligand substitution reaction. There is a cis affect on ligands; however it is so weak that it is more or less ignored 1.

Another type of trans effect involves a ? backbonding ligand. A ? backbond involves the ligand to metal ? bond and a metal to ligand ? bond. This additional ? bond provides significant energy stabilization3. When a ? backbonding ligand bonds to a metal which has two or more d electrons, the ligand is weakened where a trans ? backbonding ligand in present. This is due to the amount of ? backbonding which can occur being weakened by the competition of both ? backbonding ligands for the same d orbital. In the case of the CO ligand, the M-CO bond is weakened. However, if a ?-backbonding ligand is trans to a good ?-donating ligand, the strength of the ?-backbonding ligand to metal bond is strengthens leading to a reduction in the strength of the C-O bond 1. The more charge on the metal, the more back bonding will occur3.

The IR spectra of a metal-ligand complex can be affected by many different things. A more negative charge on the metal corresponds to more electron density shared between the metal and ligand. For CO, this means the strength of the C-O bond will be weakened and elongated. The peak will appear at a lower wavenumber on the IR spectrum2. The bond mode of the CO will also affect where its peak is on the spectra. The bonding mode with the highest energy is a terminal Co, followed by a doubly bridging CO and a triply bridging CO having the lowest energy3.

For this particular lab, we will be carrying out the following reaction

Mo(CO)6+ 1,10-phenanthroline ? Mophen(CO)4

Mophen(CO)4+ PPh3 ?Mo(phen)(PPh3)(CO)3

The structures for 1,20-phenanthroline and PPh3 can be seen in Appendix A.

Method

IR spectra of Mo(CO)6 complex, 1,10-phenanthroline, and triphenylphosphine were recorded. To prepare the first product, 1.3235 g of Mo(CO)6 was weighed out in a round bottom flask with a stop cock vent. To that flask, 0.9037 g of 1,10-phenanthroline was added with 40 mL of toluene. A stir bar was dropped into the flask and a water condenser attached to the top of the round bottom flask. The entire set up was then hooked up to an argon gas line. The solution in the round bottom was allowed to purge for 10 minutes. Once the purge was finished, the argon was allowed to flow over the reaction while it was brought to a gentle boil for one and a half hours. Once the reaction had boiled for the hour and a half time frame, the round bottom flask with the product in it was allowed to cool in an ice bath. The product was then collected in a frit and washed with diethylether. The product remained in the frit while being allowed to dry for a week in a vacuum sealed dessicator. Once the product had dried, the UV-vis and IR spectra were taken of the product using acetone and dicloromethane as the solvents respectively.

A second preparation of product was conducted by combining 0.3882 g of Mo(phen)(CO)4 was placed in a clean round bottom flask. To this 1.5737 g of triphenylphosphine and 23 mL of xylene was added. A water condenser was connected to the round bottom flask. The line was allowed to purge over the solution for about 10 minutes. Then the argon was flowed over the reaction while it refluxed for one and a half hours. After the reflux was complete, the round bottom flask and product were allowed to cool in an ice bath. The product was collected via suction filtration with a frit. The product was washed with hexane, transferred to a vial and then placed in a vacuum sealed dessicator for a week in order to dry. Once the product had dried, the IR and UV-vis spectra were collected using the same solvents as used in the intermediate complex analysis.

Data and Conclusion

The product from the first reaction was a bright red. The UV-vis spectrum of the first product, Mo(phen)(CO)4 can be seen in Figure 1. The max absorbance for the intermediate appeared at 451 nm. From the UV-vis, it can be seen that the compound was absorbing blue to green wavelengths of light. This means that it should appear orange to red in color. It appears as if there are two peaks in the UV-vis spectrum. If that is so, the first peak would correspond to d-d transition while the second would correspond to a metal to ligand charge transfer, though I feel that these peaks are not prominent to confidently say this. Figure 2 shows the IR spectrum of the intermediate complex Mo(phen)(CO)4. All of the present peaks suggest that all three CO ligands are terminal, demonstrated by their wavenumbers being in the region of 1700-2100 cm-1.

After the second reaction, it was a deep purple which appeared almost black at first sight. The UV-vis spectrum gave 453 nm at the maximum absorbance. This means that the compound is absorbing blue light wavelengths and should appear orange in color. This does not confirm the observed color of the final product. As in the intermediate complex’s UV-vis spectrum, it looks as if there are two peaks, however they are not very prominent. These two peaks would again suggest d-d transitions and metal to ligand charge transfers respectively. By estimating the peaks of the first peaks in both the intermediate and final product, it can be seen that the d-d interactions absorption peaks increase in wavelength. This means the d-d interactions have less energy in the final product than in the intermediate. The IR spectrum shows that there are some ligands which are terminal CO’s. In the final product, the last peak jumps from 2003. 71 cm-1 to 2360.48 cm-1. This would suggest that the addition of PPh3 causes less back bonding in the final product. Therefore PPh3 is not a good ? donating ligand.

Resources

1. Miessler, Gary L..Inorganic Chemistry. Sol ed. Alexandria, VA: Prentice Hall, 2003. Print.
2. Exstrom, Christopher. “Organomatellic Complexes.” Chem 430. UNK. Bruner Hall, Kearney. 17 Nov. 2009. Lecture.
3. Exstrom, Christopher .Chem 430L Laboratory Manual. Kearney NE: Department of Chemistry at University of Nebrasksa.
4. “Reaction Mechanisms – Ligand Substitutions.”LSU Chemistry. N.p., n.d. Web. 6 Dec. 2009. <chemistry.lsu.edu/stanley/webpub/4571…/chap11-substitution-rxns.doc>.