SYNTHESIS OF THE IRON-SULFUR CLUSTER COMPOUNDS [FE4S4(MECP)4](PF6)Y (Y = 0-2) - X-RAY STRUCTURE DETERMINATIONS OF FE4(MU-3-S)4(MECP)4 AND [FE4(MU-3-S)4(MECP)4](PF6)

The series of iron-sulfur cluster compounds [Fe4S4(MeCp)4](PF6)y (y = 0-2) is readily prepared by reaction of [Fe(MeCp)2(CO)2]2 with an excess of sulfur in refluxing toluene followed by a number of suitable synthesis and purification steps. The structures of Fe4S4(MeCp)4 and [Fe4S4(MeCp)4](PF6) were determined crystallographically. FeS4(MeCp)4 crystallizes in the orthorhombic system, space group Pnma, with cell dimensions a = 19.631 (1) angstrom, b = 12.453 (1) angstrom, c = 10.122 (1) angstrom, V = 2474.6 (3) angstrom 3, and Z = 4. Refinement of 169 parameters on 1562 "observed" reflections gave a final R value of 0.063 (R(w) = 0.087). The Fe4S4 moiety of the structure possesses idealized (elongated) D2d symmetry, which is in agreement with the qualitative Dahl bonding model. [Fe4S4(MeCp)4](PF6) crystallizes in the tetragonal system, space group I4, with cell dimensions a = b = 13.419 (3) angstrom, c = 7.809 (3) angstrom, V = 1406.3 (7) angstrom 3, and Z = 2. Refinement of 87 parameters on 634 "observed" reflections gave a final R value of 0.062 (R(w) = 0.073). The idealized (compressed) D2d symmetry of the Fe4S4 moiety of this structure does not agree with the Dahl model. Therefore, a slightly adjusted MO scheme is proposed, in which two energy levels are reversed.