A comprehensive vibrational assignment of the hydrocarbon features in the infrared and Raman spectra of (η2-C2H4)Os(CO)4 has been achieved with the help of data from the (13C2H4) and (C2D4) isotopologues. Close analogies with the vibrational spectra of thiirane (ethene sulfide) assist in the assignment and emphasize the metallacyclopropane nature of the (C2H4Os) group. Comparisons are made with analogous spectra from (η2-C2H4)Fe(CO)4 and from Zeise's salt, K[(η2-C2H4)PtCl3]. Taking into account the metal–surface selection rule associated with on-specular vibrational electron energy loss or reflection–absorption infrared spectra, good agreement is found between the wavenumber/intensity pattern from the a1 modes of the complex and from ethene chemisorbed on metal surfaces previously designated Type I′.