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Why would the infrared stretching frequency of $\ce{CO}$ (which is about $2140~\mathrm{cm^{-1}}$ in gaseous form) be $1982~\mathrm{cm^{-1}}$ in $\ce{[Mo(CO)_6]}$ and even lower in $\ce{[Mo(CO)3(η^6$\text{-}$Mes)]}$ ($\ce{Mes} = $1,3,5-trimethylbenzene)?

I think it may be due to the π* orbital being used in the $\ce{CO}$ ligand which reduces the bond order and lengthens the bond. The stretching frequency is proportional to the bond strength so if the bond gets weaker then the frequency is lowered.

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