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I am reading about vibrational spectroscopy of polyatomic molecules and specifically about metal complexes. What I don't understand is what happens on the vibrational modes of the ligand (e.g. water) when coordinates to a metal ion.

When the complex is formed then it doesn't make sense to speak about the vibrational modes of water because it is not anymore a triatomic molecule. But we use IR spectroscopy for identification of functional groups. So if the identity of the ligand is lost upon coordination how can we study the functional groups that have been coordinated?

In an IR absorption spectra of a complex the wavenumbers where absorption happens includes that of the ligand's (e.g. symmetric stretch of water). So that means that the water (or any other ligand) still exhibits its own vibrational modes. Shouldn't the new wavenumbers be bigger as they involve more atoms and therefore more energy is needed for the vibrational modes?

In summary, I want to understand why it is still possible to identify ligands upon coordination and why the energy of the vibrational modes is not proportional to the number of atoms. For example should we expect methane's vibrational modes to be higher in energy than the vibrational modes of pentane? An intuition based answer will be nice.

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If you recall, the infrared spectrum of a compound has several regions. One of the region is the functional group region, where you see the vibrations of a functional group of a molecule.

Then you have a fingerprint region which corresponds to the vibration of the whole molecule. As expected, this region is unique to the molecule. So, if your complex has water, you should see broad OH vibrations, and then in the fingerprint region you have more complex vibration particular to the molecule involving the bound ligands.

You should expect change in frequency of a bound ligand vs. a free ligand. Since the spectral bands are so wide in solids/liquids that the shift is not greatly visible.

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  • $\begingroup$ After coordination the water can't behave like a triatomic molecule. It is bonded to the metal ion or whatever is coordinated. Why it still exhibits its vibrational modes? $\endgroup$
    – Anton
    Aug 29 at 11:52
  • $\begingroup$ Yes, -OH can still vibrate. We are not talking about HOH. $\endgroup$
    – M. Farooq
    Aug 29 at 21:10

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