A thermochemical cycle has been constructed for the reactions of ethylene on H-presaturated Fe(100); this cycle has been used to accurately determine Fe-C2H5 and Fe-H surface bond energies of 38 +/- 4 and 59.5 +/- 2 kcal/mol, respectively. The Fe-C2H5 bond energy is considerably smaller than that measured in the gas phase. Both values are similar to those expected for mononuclear transition metal complexes, provided that the complex contains a first or second row transition metal and is not sterically crowded.