Reaction of the cubane cluster [Fe4S4(SEt)(2)((BuNC)-N-t)(6)] (7) and PhCH2SSSCH2Ph in benzene solution affords the cluster [Fe8S12((BuNC)-N-t)(12)] (6) in 60-74% yield. The structure of this cluster as [6] . 5C(6)H(6) and that of [Fe4S5(SEt)(2)((BuNC)-N-t)(6)] (8, closely related to 7) were determined by X-ray diffraction. Cluster 6 contains two cubanoid units Fe4S5 = Fe-4(mu(3)-S)(3)(mu(3):eta(2)eta(1)-S-2) bridged by two mu(2)-S ions and related by an inversion center. Certain features of the structure of 7 as revealed by 8 are retained in 6, including in each cubanoid unit two low-spin. distorted octahedral (FeS3)-S-II((BuNC)-N-t)(3) and two distorted tetrahedral (FeC)-C-III(mu(3)-S)(3)(mu(2)-S) sites and certain dimensional similarities. The formal insertion of an S(O) atom into a cubane to form a cubanoid unit disrupts one Fe-(mu(3)-S) bond and leads to dimensional changes such as to poise one iron atom in a stereochemistry and position favorable for the formation of an Fe-(mu(2)-S)-Fe bridge. The structures of iron-sulfur polycubanes are summarized; doubly sulfide-bridged double cubanes are unknown. In this family, cluster 6 is the only authenticated bis(mu(2)-S) species and thus bears a distant relationship to the P clusters of nitrogenase. Cluster 6, with four Fe(III) sites in the centrosymmetric array [Fe-2(mu(2)-S)(2)](mu(2)-S)(2), presents an exchange coupling problem not previously encountered in iron-sulfur compounds. A formal treatment of the problem based on two coupling constants for intra- (J) and intercubanoid (J ') exchange is presented. It is shown that the there is no unique solution for J,J'. Reasonable values of these parameters based on different assumptions are considered. If a recently determined J value for coupling in a [Fe-2(mu(2)-S)(2)](2+) fragment of a cubane cluster is adopted, J = 280 cm(-1 )and J' = 445 cm(-1) for 6.