Effects on Stretching Frequencies in Organometallic Complexes

If you added a negative charge to $\ce{Mo(CO)\_6}$ how would this effect the $\ce{CO}$ stretching frequency in an IR spectrum?


My thoughts are that it would lower the $\ce{CO}$ stretching frequency because the metal would be able to donate electron density to the $\pi^\*$ orbitals of $\ce{CO}$. Does this reasoning sound plausible? 


