Discussion
The kinetics of the isomerization of cis to trans - [Mo(CO)4(PEt3)2] can be followed by IR spectroscopy. Which of the following regions of the IR spectrum would you focus on to monitor the reaction ?
Answer: B
Focus on the 2000 - 1800 cm−1 region, where the carbonyl stretches lie.
Terminal CO ligands on a low-valent metal such as Mo(0) absorb between about 2050 and 1850 cm−1. The cis and trans isomers give different patterns there: cis-[Mo(CO)4(PEt3)2] (C2v) has four IR-active CO stretches, while the trans isomer (D4h) has essentially one strong band. Watching the cis bands fade and the single trans band grow follows the isomerization directly.
3500 - 3000 cm−1 holds O-H and N-H stretches, absent here. 1600 - 1400 cm−1 holds C=C and C-H bending bands, and 1200 - 800 cm−1 is the fingerprint region of the ethyl groups; neither changes clearly between the isomers.
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