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Deviating from the common growth mode of molecular films of organic molecules where the adsorbates remain intact, we observe an essentially different growth behavior for metallocenes with a low temperature scanning tunneling microscope. Ferrocene molecules adsorb dissociatively and form a two layer structure. The top layer unit cell is composed of two tilted cyclopentadienyl (cp) rings, while the first layer consists of ferrocene molecules and cp-Fe complexes. Surprisingly a fourfold symmetry is observed for the top layer while the first layer displays threefold symmetry elements. It is this symmetry mismatch which induces an incommensurability between these layers in all except one surface direction. The top layer is weakly bonded and has an antiferromagnetic ground state as calculated by local spin density functional approximation.