MIGRATORY INSERTION OF ETHYLENE INTO IRON HYDROGEN-BONDS AND BETA-HYDRIDE ELIMINATION OF ETHYL GROUPS ON H-COVERED FE(100)

Ethylene has been found to adsorb reversibly at 110 K without dissociation on Fe(100) presaturated with hydrogen. No ethane is formed following adsorption of ethylene, nor does any ethylene decompose on this surface. Desorption of ethylene at 160 K competes with the formation of adsorbed ethyl groups. Ethyl groups decompose by beta-hydride elimination with an activation barrier of 12.2 +/- 0.6 kcal/mol and a preexponential factor of 10(13.4 +/- 0.6) s-1, exhibiting a primary kinetic isotope effect, k(H)/k(D), at 219 K of 4.9 +/- 0.5. The reversible reaction of adsorbed ethylene and adsorbed deuterium to form ethyl groups leads to the incorporation of up to four D atoms in the ethylene evolved during temperature-programmed reaction. The activation energies determined here indicate that barriers to formation and beta-hydride elimination of adsorbed ethyl groups agree with those observed for transition-metal complexes only after the energetics of structural rearrangements in the complex are removed.