In literature the stretching mode of $\ce{CO}$ is often said to be the diagnostic criteria for strength of back-donation by virtue of weakening the $\ce{CO}$ bond. However, when speaking about homoleptic carbonyls, the vibrations of individual $CO$ ligands interact, forming symmetric and asymmetric modes with considerably different frequencies.

For example, let's say I want to compare strength of back-donation in $\ce{Cr(CO)6}$ and $\ce{Fe(CO)5}$ and compare it with a free molecule?

  • $\ce{Cr(CO)6}$ has modes $2119 (A_{1g}), 2027 (E_{g}), 2004(T_{1u})\ cm^{-1}$
  • $\ce{Fe(CO)5}$ has modes $2121 (A_{1}^{'}), 2041 (A_{1}^{'}), 2034 (A_{2}^{''}), 2013 (E_{1}^{'})\ cm^{-1}$
  • free $\ce{CO}$ vibrates at $2143 cm^{-1}$ )

What modes I should use for comparison?



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