A functional model for nitrogenase is currently sought in our laboratory in reactivity studies using various single cubane clusters that possess the [ MFe3S4]" + core, (M = Mo, n = 3; M = V, n = 2). These clusters are used as catalysts for the reduction of enzymatically relevant substrates. Substrates such as hydrazine and acetylene are catalytically reduced by (NEt4)2[(C14cat) (CH3CN) MoFe3S4C13 ], I, to ammonia and ethylene respectively, in the presence of added protons and reducing equivalents. Hydrazine also is catalytically reduced by the (NEt4) [ (DMF) 3VFe3S4C13 ] cubane under similar conditions. Gas chromatography was employed to monitor the reduction of acetylene to ethylene and a trace of ethane. Catalysis in excess of 100 turnovers (for hydrazine reduction) and in excess of 15 turnovers (in acetylene reduction) has been demonstrated over a period of 24 h. A study of the acetylene reduction reveals saturation kinetics to be operating at high substrate concentrations. A variable temperature kinetic study of acetylene reduction shows a moderate activation energy (E~t = 9( 1 ) kcal mol-l) but a large entropy of activation ( A S ~ = -32 (2) cal K-l mol-~ ) which extrapolates to a significant Gibbs free energy ( A G* = 19 ( 1 ) kcal mol -~ ). These results are consistent with an ordered transition state. Considerable evidence has been amassed which directly implicates the Mo and V atoms as the primary catalytic sites. Replacement of the Mo or V bound, kinetically labile, solvent molecules with non-labile ligands acts to suppress the observed rates of reaction. The Fe sites on I are totally inactive in the reduction of hydrazine, however they have been found to effect acetylene reduction albeit at a markedly reduced rate compared to the Mo site. Catalyst integrity has also been demonstrated by a variety of techniques, primarily EPR spectroscopy which identifies the characteristic S = 3/2 signals of the Mo and V cubanes after at least 18 h reaction time.