Singly bridged double cubane clusters containing Fe4S4 and/or MoFe3S4 cores and linked by an Fe-S-Fe unit bear an interesting compositional similarity to the cofactor of nitrogenase and constitute a nearly unexplored class of clusters. We have expanded recently our investigation of the prototypic cluster {[(Meida)MoFe3S4Cl2](2)(mu(2)-S)}(4-) (6) (Huang et al.; J. Am. Chem. Soc. 1997, 119, 8662) to other types {[(Meida)MoFe3S4L2](2)(mu(2)-S)}(4-) (L = EtS-, ArO- (8-10)) (Meida = N-methylimidodiacetate(2-)). Clusters are formed by coupling reactions of the single cubanes [(Meida)MoFe3S4L3](2-) (L = Cl- (1), ArO- (3-5) at their substitutionally labile iron sites with primarily Li2S. Using 2 NaOEt/H2O as an oxide source in Me2SO, the ore-bridged clusters {[(Meida)MoFe3S4L2](2)(mu(2)-O)}(4-) (L = Cl- (13), ArO- (14, 15)) have been prepared. All double cubanes were obtained as a mixture of as many as four diastereomers. Double cubane structures were established by NMR and electrochemical criteria. Both the sulfido- and oxo-bridged clusters undergo terminal ligand substitution reactions with bridge retention. Oxo bridges are much more susceptible than sulfide bridges to protic cleavage with water, (Et3NH)Cl, ArOH, and HS-. Reaction of 13 with hydrosulfide affords a second route to 6. These and other results define a set of reactions that are regiospecific at either the terminal ligand or bridge sites and provide a basis for future reactivity manipulation of symmetrical and unsymmetrical bridged double cubanes.