Source: Tamara M. Powers, Department of Chemistry, Texas A&M University

Metal carbonyl complexes are used as metal precursors for the synthesis of organometallic complexes as well as catalysts. Infrared (IR) spectroscopy is one of the most utilized and informative characterization methods of CO containing compounds. Group theory, or the use of mathematics to describe the symmetry of a molecule, provides a method to predict the number of IR active C-O vibrational modes within a molecule. Experimentally observing the number of C-O stretches in the IR is a direct method to establish the geometry and structure of the metal carbonyl complex.

In this video, we will synthesize the molybdenum carbonyl complex Mo(CO)₄[P(OPh)₃]₂, which can exist in the cis- and trans-forms (Figure 1). We will use group theory and IR spectroscopy to determine which isomer is isolated.

Figure 1. The cis- and trans-isomers of Mo(CO)₄[P(OPh)₃]₂.

1. Setup of the Schlenk Line (for a more detailed procedure, please review the "Schlenk Lines Transfer of Solvent" video in the Essentials of Organic Chemistry series). Schlenk line safety should be reviewed prior to conducting this experiment. Glassware should be inspected for star cracks before use. Care should be taken to ensure that O₂ is not condensed in the Schlenk line trap if using liquid N₂. At liquid N₂ temperature, O₂ condenses and is

Figure 5. IR of Mo(CO)₄[P(OPh)₃]_2.

Solution IR in saturated hydrocarbon (cm^-1): 2046 (s), 1958 (s), 1942 (vs).
The fourth resonance can only be seen under high-resolution conditions. Therefore, it is possible, as in this case, that only 3 of the 4 resonances are observed.
Based on the obtained IR, we can conclu

In this video, we learned how to use group theory to predict the number of IR active vibrational modes in a molecule. We synthesized the molecule Mo(CO)₄[P(OPh)₃]₂ and used IR to determine which isomer was isolated. We observed that the product had three C-O vibrations in its IR spectrum, which is consistent with the cis-isomer.

Group theory is a powerful tool that is used by chemists to not only predict IR active vibrational modes, but also vibrationa

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