As the metal atoms are protected by the methyl groups, ligand exchange of CH3 to NH2 cannot occur directly. Instead, a methyl group must desorb from the surface, leaving a vacancy onto which NH3 can adsorb, in a SN1 like mechanism. While adsorbing, a H atom can be transferred from the NH3 to the released methyl group allowing it to be released as methane leaving NH2 to adsorb onto the vacant site.