A theoretical analysis of the chemisorption of ethylene on a platinum surface is presented as a function of the metallic coordination of the platinum surface atom. Adsorption sites with coordinations ranging from 9 to 3 are considered and modeled by clusters of size between 71 and 114 atoms, with a special emphasis on step and kink sites. From extended-Huckel calculations, the ethylene molecule is shown to adsorb in a di-sigma mode for sites with high metallic coordinations while the pi chemisorption becomes more stable for coordinations 7 and below. The binding energy of the molecule is increased when the surface atom coordination is reduced. These changes are explained by a strong reduction of the four-electron repulsion between occupied molecular orbitals of ethylene and filled states of the cluster for sites with low metal coordination. This sensitivity of the molecule adsorption on the structure of the surface could have important implications on the ethylene reaction pathway.