The transition metal dioxide-ethylene complexes (eta(2)-C2H4)MnO2 and (eta(2)-C2H4)FeO2 have been prepared by the reactions of transition metal dioxide molecules (MnO2 and FeO2) with ethylene in solid argon The metal dioxide molecules were prepared by the reactions of laser-ablated metal atoms with dioxygen. The (eta(2)-C2H4)MnO2 and (eta(2)-C2H4)FeO2 complexes were characterized by matrix isolation infrared absorption spectroscopy as well as density functional calculations. Both complexes were predicted to have low spin ground states, having C-2v symmetry with the MC2 plane perpendicular to the MO2 plane. (C) 2003 Elsevier B.V. All rights reserved.