The all-ferrous [Fe4S4](0) state has been demonstrated in the fully reduced Fe protein of the Azotobacter vinelandii nitrogenase complex. We seek synthetic analogues of this state more tractable than the recently prepared but highly unstable cluster [Fe4S4(CN)(4)](4-) (Scoff, Berlinguette, Holm, and Zhou, Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9741). The N-heterocyclic carbene 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene ((Pr2NHCMe2)-N-i) has been found to stabilize the fully reduced clusters [Fe(8)S8(()Pr(2)(i)NHCMe(2))(6)] (4) and [Fe4S4((Pr2NHCMe2)-N-i)41 (5), which are prepared by cluster assembly or phosphine substitution of FenSn (n = 8, 16) clusters. Cluster 4 is also obtained by reaction of the carbene with all-ferrous [Fe7S6(PEt3)(5)Cl-2] (3) and cluster 5 by carbene cleavage of 4. Detailed structures of 3 (monocapped prismatic), 4, and 5 are described; the lafter two are the first iron-sulfur clusters with Fe-C sigma bonds. Cluster 4 possesses the [Fe-8(mu(3)-S)(6)(mu(4)-S)(2)] edge-bridged double cubane structure and 5 the cubane-type [Fe4(U3-S)41 stereochemistry. The all-ferrous formulations of the clusters are confirmed by X-ray structure parameters and 57 Fe isomer shifts. Both clusters are stable under conventional aprotic anaerobic conditions, enabling further study of reactivity. The collective properties of 5 indicate that it is a meaningful synthetic analogue of the core of the fully reduced protein-bound cluster.