The reactions of transition metal dioxides (MnO2 and FeO2) with acetylene molecules have been studied using matrix isolation infrared absorption spectroscopy. The metal dioxide molecules were prepared by the reactions of laser-ablated metal atoms with dioxygen. In solid argon, the MnO2 and FeO2 molecules reacted with C2H2 to form the (eta(2)-C2H2)MnO2 and (eta(2)-C2H2)FeO2 complexes spontaneously on annealing. Both complexes were predicted to have low-spin ground states ((1)A(1) for (eta(2) -C2H2)FeO2, (2)A, for (eta(2)-C2H2)MnO2) with C-2nu symmetry. In addition, evidence is also presented for the formation of Mn(H)CCO and Fe(H)CCO molecules upon UV-visible irradiation. The product absorptions were identified by isotopic substitution as well as density functional calculations of isotopic frequencies.