The complex ethylidyne tricobalt nonacarbonyl is used as a model compound for ethylidyne chemisorbed on metal surfaces. In the present work. we have used the combination of infrared, FT-Raman and inelastic neutron scattering spectroscopies to test and extend previous assignments. We have located the missing mode, the torsion about the Co3C-CH3 bond. This occurs at 208 cm(-1) and is highly mixed with the Co3C-CH3 bending mode. DFT calculations confirm that the frequency is a property of the molecule and is not imposed by solid-state effects. Accordingly, we would expect it to occur on a metal surface close to the frequency found in the complex. The FT-Raman spectra also show all of the carbonyl stretching modes for the first time and these are assigned by comparison to the DFT calculations. This also illustrates the maturity of the ab initio DFT methods in the prediction of INS spectra of compounds containing heavy elements.