# Comparing bond strength by IR frequency: symmetrical modes

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?