# Origin of broad CO stretch in infrared spectrum of tricobalt cluster

I'm quite confident that the peaks around $\pu{2000 cm^{-1}}$ are $\ce{CO}$ stretches. But I cannot find any reference on that massive broad stretch following the $\ce{CO}$ stretches. Does it have anything to do with high symmetry?

• If there is expected to a range if different CO frequencies in your molecules I would clearly expect to see a series of discrete bands not a broad featureless absorption so my first action, before worrying about what causes this, would be to purify the compounds again and check that some other instrumental artefact has not occurred. – porphyrin May 27 '18 at 9:01
• I agree. For example, I found it easy to get spectra with poor lineshapes like this on a benchtop ATR; thin-film on salt plates was better, but KBr pellets were best. This is not the kind of broadening you would see in paramagnetic NMR, but probably has to do with sample preparation. – pentavalentcarbon May 28 '18 at 1:26
• Yea, that's a good point. It looks like I got some solvent or starting materials stuck between the clusters. But unfortunately I did not have enough lab time to recrystallise it. – Van Gouch Jun 2 '18 at 11:19

I found an reliable article (http://www.ilpi.com/organomet/carbonyl.html) describing wide spectrum of metal carbonyls. Since Chemistry SE won't open that website, I have attached scheme with a range of carbonyl stretching frequencies in the article for your benefits:

The IR stretching of carbonyl can be varied based on the structure. According to the website:

In the IR, typical stretching frequencies are: (i) Uncoordinated or "free" $\ce{CO}$: $\pu{2143 cm^{-1}}$; (ii) Terminal $\ce{M-CO}$: $\pu{2125}$ to $\pu{1850cm^{-1}}$; (iii) Doubly bridging ($\mu_2$): $\pu{1850}$ to $\pu{1750 cm^{-1}}$; (iv) Triply bridging ($\mu_3$): $\pu{1675}$ to $\pu{1600 cm^{-1}}$; and (v) Semibridging: somewhere between terminal and $\mu_2$.

In literature, most tricobalt clusters were discussed included with other metal cations. For example: (a) Rational synthesis of tricobalt-molybdenum and -tungsten butterfly clusters with alkyne ligands in Journal of Organometallic Chemistry, 2002, 659(1-2), 142-150 (https://doi.org/10.1016/S0022-328X(02)01720-5) and (b) Clusters as Ligands. 5. Tricobalt Cluster Alkoxycarboxylates of Titanium and Zirconium Exhibiting Novel Structures and Properties in Organometallics, 1997, 16(24), 5289–5301 (https://pubs.acs.org/doi/abs/10.1021/om970837b).

You can also read: A. S. Goldman, K. Krogh-Jespersen, Why Do Cationic Carbon Monoxide Complexes Have High $\ce{C-O}$ Stretching Force Constants and Short $\ce{C-O}$ Bonds? Electrostatic Effects, Not $\sigma$-Bonding, J. Am. Chem. Soc., 1996, 118, 12159-12166 (https://cdn-pubs.acs.org/doi/10.1021/ja960876z).

Since you have completely described preparation procedure of your tricobalt cluster, I assume the broasness of carbonyl absorptions are due to different structural aspects of carbonyl on $\ce{Co}$ and other metals in the cluster. It is also possibility that you have a mixture of clusters. Note that range of $\nu_\mathrm{CO}$ could be $\pu{2100}$ to $\pu{1300 cm^{-1}}$, based on stereo arrangement (see above references).

• This is nice and all, but I'm not sure if you're addressing the main point of the question, which is to do with the unusually broad peak at $2000\text{–}1000~\mathrm{cm^{-1}}$. – orthocresol May 27 '18 at 0:25
• @orthocresol: Please see added information in late edition. – Mathew Mahindaratne May 27 '18 at 2:16