The chemisorption modes of ethylene (ethene) on four Pt-group electrodes (platinum, palladium, rhodium, and iridium) and on gold in acidic aqueous solution are explored by means of surface-enhanced Raman spectroscopy (SERS), the former surfaces being prepared as ultrathin films on a SERS-active gold substrate. The primary objective is to compare the periodic trends in electrode-ethylene bonding with the extensive information available for metal surfaces in gaseous and vacuum environments. Each surface yielded vibrational spectra that indicate the extensive presence of pi -bound molecular ethylene, primarily from the appearance of coupled C=C stretch [nu (C=C)] and CH2 Scissors [delta (s)(CH2)] vibrations at 1495-1540 and 1190-1275 cm(-1), the frequencies depending on the metal. The marked (ca. 70-150 cm(-1)) coordination-induced redshifts observed for these vibrations, in the sequence Au < Pd < Rh < Pt < Ir, indicate the occurrence of increasing metal-ethylene 2 pi* back-donation. The importance of this bonding interaction is indicated further from positive frequency-potential (i.e., Stark tuning) slopes for these vibrations that increase (up to ca. 20 cm(-1) V-1) in a similar metal-dependent sequence. The flat pi -bound ethylene orientation is also consistent with the observed absence of sp(2) C-H bands on the basis of Raman surface selection rules. On platinum, and especially rhodium and iridium, however, vibrational bands at 2880-2890, 1340, and 460-470 cm(-1) diagnose the additional presence of chemisorbed ethylidyne (equivalent toC-CH3) being assigned to sp(3) C-H stretching, CH3 deformation, and metal-carbon stretching vibrations, respectively. Ethylidyne was formed from chemisorbed ethylene increasingly toward lower potentials. Once formed, chemisorbed ethylidyne is stable over a markedly wider range of electrode potentials than pi -bound ethylene, indicating a greater resistance of the former to both electrooxidation and electrohydrogenation.