Using high resolution neutron powder diffraction and neutron spectroscopy, ISIS scientists have been able to re-examine the structure and vibrational spectroscopy of iron pentacarbonyl, Fe(CO)5. Unexpectedly, their results uncovered a new phase, and show that the true behaviour of the molecule is different to that predicted by computer modelling.
Although Fe(CO)5 was the second metal carbonyl to be discovered, its unusual symmetry means that its vibrational structure continues to be under investigation. In this study, published in JACS, ISIS researchers Stewart Parker and Dominic Fortes carried out a comprehensive study of both the structure and the vibrational spectroscopy of the compound across a wide temperature range covering both the solid and liquid forms.
By using neutron diffraction on the HRPD instrument, they discovered a previously un-reported phase transition. In addition, this transition proved to be ferroelastic, thought to be caused by the Van de Waals strain caused by the bond lengths shrinking on cooling. With this structural knowledge gained, they moved on to studying the molecule's vibrations.
Because neutron spectroscopy is not limited by the selection rules present in optical spectroscopy, by using inelastic neutron scattering (INS), they were able to observe all of the internal modes of vibration in the molecule for the first time. By combining their INS results with additional optical spectroscopy collected at the Research Complex at Harwell, they were able to assign correctly all the vibrational modes, finding that the previous literature was incorrect in multiple areas.
The biggest surprise for Stewart and Dominic happened when they went to compare their results to those calculated using Density Functional Theory (DFT). Although during previous studies of metal hexacarbonyls, the DFT calculations correctly matched the spectra, this was not the case for Fe(CO)5.
“We still don't understand why this is the case," adds Stewart; “The accuracy of the molecular bond distances and angles was similar to that for the hexacarbonyls, but in this instance the calculations do not fit the results."
The full paper can be found at DOI: 10.1021/jacs.2c01469