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:

enter image description here

As the C-O bonds both enlargen, the dipole moments should cancel? Or am I missing something?


1 Answer 1


You are completely right in your thinking. Only assymetric stretching can be seen in IR spectroscopy, and this is exactly reflected by the diagram. The left part shows stretching above and below Me center, these cancel out; but on the rear there are 2 CO molecules contributing to the change of dipole moment.

  • $\begingroup$ Yeah true but that is not what the diagram describes 🤔 I think the other two CO’s are depicted as not moving $\endgroup$
    – Mäßige
    Oct 29 at 14:54

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