Structure and Spectroscopy of Iron Pentacarbonyl, Fe(CO)₅

We have re-investigated the structure and vibrational spectroscopy of the iconic molecule iron pentacarbonyl, Fe(CO)₅, in the solid state by neutron scattering methods. In addition to the known C2/c structure, we find that Fe(CO)₅ undergoes a displacive ferroelastic phase transition at 105 K to a P1̅ structure. We propose that this is a result of certain intermolecular contacts becoming shorter than the sum of the van der Waals radii, resulting in an increased contribution of electrostatic repulsion to these interactions; this is manifested as a strain that breaks the symmetry of the crystal. Evaluation of the strain in a triclinic crystal required a description of the spontaneous strain in terms of a second-rank tensor, something that is feasible with high-precision powder diffraction data but practically very difficult using strain gauges on a single crystal of such low symmetry. The use of neutron vibrational spectroscopy (which is not subject to selection rules) has allowed the observation of all the fundamentals below 700 cm^-1 for the first time. This has resulted in the re-assignment of several of the modes. Surprisingly, density functional theory calculations that were carried out to support the spectral assignments provided a poor description of the spectra.