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With reference to this graph, my professor stated the following: "Since the IR band for the bridging carbonyls is single, it implies that the bridging carbonyls are oriented linearly. Since we have two bands for the terminal bonds, they are not collinear."
Precisely how and why?

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    $\begingroup$ This has to do with symmetry. If two CO groups are symmetric (i.e. interchangeable with a symmetry operation) then the vibrations of those stretches will couple to each other, and form a symmetric and antisymmetric band. Now the symmetric band would be invisible in the IR spectrum, if the CO are aligned perfectly opposite to each other, because there would be no change in the dipole moment in the symm. stretch... $\endgroup$
    – S R Maiti
    Commented Dec 27, 2021 at 14:39
  • $\begingroup$ ...however, the asymmetric stretch is usually always visible because there is a dipole moment change. If you can see both symm. and asymm. peaks that means your CO ligands are not perfectly opposite to each other i.e. not collinear. This is a kind of simplified analysis that works for pairs of CO ligands, for higher symmetry (i.e. three fold) you need to use more rigorous group theory. $\endgroup$
    – S R Maiti
    Commented Dec 27, 2021 at 14:42
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    $\begingroup$ @SRMaiti, why not turn your comments into an answer? $\endgroup$ Commented Dec 28, 2021 at 13:30

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