it's been a few years since I took inorganic chemistry and was starting to refresh some of the concepts. What I didn't really understood when I reread was the following: How to properly determine the number of vibrational modes of CO in carbonyl complexes.
I always understood the vibration of CO as the streching of the C-O-bond as in the diatomic molecule, disregarding the M-C bond; so only the internuclear distance of C and O changes and so does the dipole moment. That is the premise; is that correct?
Also, I recall that one can only see the asymmetric strech of CO when bound in a trans-fashion to the metal center, because the symmetric strech won't change the dipole moment. How come then that this strech on the left is IR-active:
As the C-O bonds both enlargen, the dipole moments should cancel? Or am I missing something?