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I am struggling to rationalise why there are two Infrared vibrational frequencies ($\pu{2082 cm^-1}$ and $\pu{2019 cm^-1}$) for the stretching of the terminal $\ce{CO}$ ligands of $\ce{Fe2(CO)9}$. From my understanding of the structure, all 6 terminal $\ce{CO}$ ligands are equivalent and thus I w...