[CopperSmurf]

How can I predict which octahedral complex has a higher vCO stretching frequency (in infrared), qualitatively speaking of course?
i.e. Cr(CO)₆ vs W(CO)₆

[stewie griffin]

Well one place to start is to think about back-bonding from the metal. The more electron rich the metal (say a negatively charged complex versus a neutral complex) the more the metal can back-donate electrons to the carbonyl. The more back-bonding means a weaker C-O bond in the carbonyl (and thus less back-bonding means a stronger C-O bond).