The reactivity modeling of the nitrogenase enzyme is currently being pursued in our laboratory using various single cubanes possessing the [MFe(3)S(4)](3+) core as catalysts for the reduction of enzymatically relevant substrates (M = Mo, n = 3; M = V, n = 2). One such substrate, acetylene, is catalytically reduced by (NEt(4))(2)[(Cl-4-cat)(CH3-CN)MoFe3S4Cl3] (I : Cl(4-)cat = tetrachlorocatecholate dianion) to ethylene and small amounts of ethane in the presence of added protons (lutidine hydrochloride) and reducing equivalents (cobaltocene). Gas chromatography was employed to monitor the progress of the reaction. Catalysis in excess of 15 turnovers has been demonstrated over a period of 24 h. A kinetics study reveals saturation kinetics to be operating at high substrate concentrations; however, at lower, optimum substrate levels initial reaction velocities upsilon(0) may be obtained and further used in a double reciprocal plot, upsilon(0)(-1) vs [C2H2](-1), to determine K-m approximate to 17.9 mM and V-max approximate to 1.1 x 10(-4) M/min. A study of this reaction at five temperatures indicates a moderate activation energy (E(ast) = 9(1) kcal mel(-1)) but a large entropy of activation (Delta S+ = -32(2) cal K-1 mel(-1)) which extrapolates to a significant Gibbs free energy (Delta G(+) = 19(1) kcal mel(-1)). The large negative Delta S+ is consistent with an ordered transition state. Considerable evidence has been amassed which directly implicates the Mo atom of I as the primary catalytic site. Replacement of the Mo bound solvent molecule of I with non-labile ligands acts to suppress the observed rate of reaction. In addition, the Fe sites on the catalyst have been found to effect substrate reduction albeit at a markedly reduced rate compared to the heterometal. Catalyst integrity has also been demonstrated by a variety of techniques, primarily EPR spectroscopy which identifies the characteristic S = 3/2 Signal of cubane I after at least 18 h of reaction time. The geometry of addition across the substrate triple bond has been shown to be cis by the use of C2D2 which results in the formation of cis-1,2-C2H2D2 as the predominant isomer. Implications for the action of the nitrogenase enzyme are also addressed.