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I am struggling to rationalise why there are two infrared vibrational frequencies ($\pu{2082 cm^-1}$ and $\pu{2019 cm^-1}$) for the stretching of the terminal $\ce{CO}$ ligands of $\ce{Fe2(CO)9}$. From my understanding of the structure, all six terminal $\ce{CO}$ ligands are equivalent and thus I would expect only one vibrational frequency. Why there are two?

The structure is as follows:

structure of Fe2(CO)9

The bridging ligands are accounted for by a vibrational frequency at $\pu{1829 cm^-1}$.

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    $\begingroup$ Do you have an illustration about the molecular structure? Though about an hexacarbonyl iron complex, which was described earlier on chemistry.se here, degeneration of molecular symmetry may contribute to the presence of multiple IR absorption bands. $\endgroup$
    – Buttonwood
    Jan 11 at 17:55
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    $\begingroup$ Note, chemical information may be advantageously formatted using on ChemSE with mhchem. Take moment to familiarize with this. You are encouraged to use it in the body of questions, answers, and comments. Because it is something special not all web browsers understand well, do not use it in the title of questions or answers. $\endgroup$
    – Buttonwood
    Jan 11 at 17:57
  • $\begingroup$ I have added the structure to the original post and accounted for the bridging ligands. $\endgroup$
    – kocall4
    Jan 11 at 18:07
  • $\begingroup$ 2082 cm^−1 = 63420 GHz = 63.420 THz. 2019 cm^−1 = 60530 GHz = 60.530 THz. If it was light, the wavelengths would approx. be 4.73 µm and 4.95 µm, respectively. $\endgroup$ Jan 12 at 16:39

2 Answers 2

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Despite the title of the publication, Fletcher et al. both present an IR spectrum of diiron nonacarbonyl in a matrix of ($\ce{Ar}$ + 10% $\ce{CO}$, recorded at $\pu{15 K}$) in the $\nu(\ce{C-O})$ region of about $2080 \dots \pu{1820 cm^-1}$ (left to right hand side) as below:

enter image description here

(composite of two illustrations by Fletcher et al.)

Given the data recorded, the authors' assignment is (emphasis in the original publication):

«The spectrum of $\ce{Fe2(CO)9}$ shows two striong bands in the terminal $\ce{C-O}$ stretching region, ~$\pu{2000 cm^-1}$, as expected ($a''_2$ + $e'$) for a $D_{3h}$ structure, 1. In the bridging $\ce{C-O}$ stretching region, ~$\pu{1840 cm^-1}$, there is a single band, again as predicted, $e'$

Reference:

Fletcher, S. C.; Poliakoff, M.; Turner, J. J. Structure and reactions of octacarbonyldiiron: An IR spectroscopic study using carbon-13 monoxide, photolysis with plane-polarized light, and matrix isolation. Inorg. Chem. 1986, 25, 3597–3604; doi 10.1021/ic00240a014.


Since there is an entry about this compound in Wikipedia, you have access to the/a CAS registry number (here, 15321-51-4). If your school has the corresponding subscriptions, you may use this as a search criterion e.g., in Reaxys (by Elsevier) or SciFinder (by ACS) to identify more recent publications with an assigned IR spectrum of said compound. You may complement the search with checking the publications citing the work by Fletcher et al. (landing page of the publication already lists 63 works known to the journal).

There is a good chance Martyn Poliakoff, co-author of the paper, and Sir Martyn Poliakoff, a chemist at University of Nottingham and still engaged in popularization of chemistry on youtube/The periodic videos, are the very same person.

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It basically comes down to molecular symmetry and since Buttonwood provided an answer just seconds ago, I'll leave the following as visualisations for it.

An optimisation of $\ce{Fe2(CO)9}$ at the DF-B97D3/def2-SVP level of theory results in a D3h symmetric molecule.

This in term results in degenerate vibrations (E') for the bridging $\ce{CO}$ at $\pu{1922 cm-1}$.

mode 46 mode 47

There are degenerate vibrations (E') for the terminal ligands at $\pu{2059 cm-1}$

mode 51 mode 52

Furthermore, there is another vibration (A2'') at $\pu{2085 cm-1}$.

mode 53

Please bear in mind that this is an instructional answer (if it really is one) and the level of theory chosen for it is simple enough to make some approximate observations. Without further checking, the modes are surprisingly close to the values you have posted.

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    $\begingroup$ What a coincidence ... +1. $\endgroup$
    – Buttonwood
    Jan 11 at 22:52
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    $\begingroup$ @Buttonwood I am very grateful for your answer, it saved me a lot of work! ;) $\endgroup$ Jan 11 at 22:54
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    $\begingroup$ The publication is mentioned among the ones in the English edition of Wikipedia. By title of the publication, I was on the rim to suggest the OP something in lines of «search by CAS number in Reaxys/Scifinder (if your school has access to them)» since the other ones (incl. the Russian edition) didn't tell much. Until I read the paper... $\endgroup$
    – Buttonwood
    Jan 11 at 22:59
  • $\begingroup$ In other words, infrared spectra convey information about molecular vibrations, which are not necessarily well characterized as vibrations of individual ligands or bonds (though that impression can be taken away from some undergraduate courses). That the six terminal CO ligands are expected to to be equivalent with respect to the average point symmetry of the molecule does not imply that there can be only one vibrational mode to which their stretches are a major contributor. $\endgroup$ Jan 12 at 19:29
  • $\begingroup$ @JohnBollinger I'm not sure if isotopic labeling (on behalf of CO) may shed information if the terminal CO stay in place, or interchange with the µ CO bridging. On the other hand, copy-paste benzaldehyde (c1ccccc1C(=O)C) into MolCalc, request optimization, calculate properties / tab «Vibrational Frequencies». Already the one around $\pu{155 cm^-1}$ conveys the impression that wiggling the methyl group influences the relative orientation of carbonyl group and phenyl ring. Sadly, JSmol used for the visualization only reports steady state distances/angles here. $\endgroup$
    – Buttonwood
    Jan 12 at 20:21

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