The structures of the P cluster and cofactor cluster of nitrogenase are well-defined crystallographically. They have been obtained only by biosynthesis; their chemical synthesis remains a challenge. Synthetic routes are sought to the P cluster in the P-N state in which two cuboidal Fe3S3 units are connected by a mu(6)-S atom and two Fe-(mu(2)-S-Cys)-Fe bridges. A reaction scheme affording a Mo2Fe6S9 cluster in molecular form having the topology of the P-N cluster has been devised. Reaction of the single cubane [(Tp)MoFe3S4Cl3](1-) with PEt3 gives [(Tp)MoFe3S4(PEt3)(3)](1+) (2), which upon reduction with BH4- affords the edge-bridged all-ferrous double cubane [(Tp)(2)Mo2Fe6S8(PEt3)(4)] (4) (Tp = tris(pyrazolylhydroborate(1-)). Treatment of 4 with 3 equiv of HS- produces [(Tp)(2)Mo2Fe6S9(SH)(2)](3-) (7) as the Et4N+ salt in 86% yield. The structure of 7 is built of two (Tp)MoFe3(mu(3)-S)(3) cuboidal fragments bridged by two mu(2)-S atoms and one mu(6)-S atom in an arrangement of idealized C-2 symmetry. The cluster undergoes three one-electron oxidation reactions and is oxidatively cleaved by p-tolylthiol to [(Tp)MoFe3S4(S-p-tol)(3)](2-) and by weak acids to [(Tp)MoFe3S4(SH)(3)](2-). The cluster core of 7 has the bridging pattern [Mo2Fe6(mu(2)-S)(2)(mu(3)-S)(6)(mu(6)-S)](1+) with the probable charge distribution [Mo23+Fe52+Fe3+S9](1+). Cluster 7 is a topological analogue of the P-N cluster but differs in having two heteroatoms and two Fe-(mu(2)-S)-Fe instead of two Fe-(mu(2)-S-Cys)-Fe bridges. A best-fit superposition of the two cluster cores affords a weighted rms deviation in atom positions of 0.38 Angstrom. Cluster 7 is the first molecular topological analogue of the P-N cluster. This structure had been prepared previously only as a fragment of complex high-nuclearity Mo-Fe-S clusters.