EHT calculations on nickel(111) and nickel aluminium two-layer clusters show that an electron transfer takes place from Al to Ni 3d orbitals. Interaction of Ni11-xAlx and Ni10-xAlx with ethylene and formaldehyde, respectively, has been analyzed in terms of fragments, showing that both Π and Π* orbitals of ethylene interact with the cluster d band, while only the Π* orbital of H2CO significantly interacts with the cluster. Residual aluminium in the nickel catalyst destabilizes the four-electron interaction between solid and ethylene, so the chemisorption step becomes less favourable.