Molecular ethylene adsorption on Rh(111) and Rh(100) between 90 and 200 K has been studied by thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), and high-resolution electron energy loss spectroscopy (HREELS). The HREEL vibrational spectra for molecularly adsorbed ethylene on these surfaces are interpreted by using the Dewar-Chatt-Duncanson model for ethylene coordination to metal atom(s); gas-phase ethylene and gauche 1,2-dibromoethane are used to model the coordination extremes. It is shown that the traditional CH2 functional group modes are coupled in adsorbed ethylene and do not adequately describe the normal modes of vibration. However, the C-C stretching force constant in adsorbed ethylene can be estimated from the deuteriated ethylene vibrational frequencies, and a general correlation between deuteriated vibrational frequency and adsorbed C-C force constant is proposed. Using this correlation together with empirical correlations between force constant and bond length derived for gas-phase molecules, we estimate C-C bond orders of 1.5 and 1.2 for ethylene adsorbed on Rh(111) and Rh(100), respectively. These results indicate that on both surfaces chemisorbed ethylene is strongly distorted from its gas-phase geometry. © 1988 American Chemical Society.