[Chaggi]

The vibrational frequencies of molecules are measured to probe information about bonding, which is the basis of infrared (IR) spec. In IR spec, the bond strength (order) correlates with the frequency, with higher frequency resulting from higher bond orders. The vibration frequency of the C-O triple bond in carbon monoxide is 2143 cm(-1). However, when CO is bound to chromium in the coordination compound Cr(CO)6, the vibrational frequency drops to 2000 cm (-1).

Why?

[Yggdrasil]

Look up pi backbonding in an inorganic chem book.

[Chaggi]

Thank you! I will do so. Will post if I have any more questions!