Clusters of the general type {[MFe3S4(SR)2(cat)]2(μ2-SR)2}4- (cat = catecholate) have previously been shown to contain two heterometal cubane-type cores [MFe3S4]3+ (M = Mo, W) linked by two Fe−S(R)−M bridges such that the cluster has idealized centrosymmetry. These clusters resemble the P-clusters of nitrogenase in that both have two μ2-SR bridges between cubane (or cuboidal) halves of the cluster. When dissolved in a strongly coordinating solvent or treated with a suitable ligand L1-,0, the bridges are cleaved to afford the single cubanes [MFe3S4(SR)3(cat)L/(solv)]2-,3-. Because of their relation to P-clusters, the stability and electron transfer properties of the double cubanes have been examined in solution. 1H NMR spectroscopic and cyclic voltammetric properties of acetonitrile solutions prepared from Et4N+ salts of clusters with R = Et and M = Mo, W are entirely consistent with the presence of bridged double cubanes in solution. Key observations include different chemical shifts of bridging and termi...