Anaerobic reaction of equimolar FeX2 and RS” in acetonitrile affords the trinuclear complexes (with X = Cl and R = Ph (4a), p-tolyl (4b), 2,6-Me2C6H3(4d), and X = Br, R = Ph (4c)) isolated as Et4N+ salts. These species constitute the fourth known type of discrete Fe(II) thiolates, the others being [Fe(SR)4]2-, and The following crystallographic results were obtained (crystal system; space group; cell parameters; Z; unique data (I > 3σ(I)); R (%)). (Et4N)3 (4a): monoclinic; C2/c; a = 24.500 (6) A, b = 13.488 (3) A, c = 18.217 (5) A, β = 110.68 (2)°; 4; 2679; 4.9. (Et4N)3a = 15.07 (1) A, b = 22.38 (3) A, c = 20.46 (3) A, β = 91.5 (1)°; 4, 3311; 9.7. (Et4N)3 (4c)-MeCN: hexagonal, P62c; a = b = 13.970 (4) A, c = 18.274 (3) A; 2; 890; 5.7. The reaction system FeBr2+ NaSCH2Ph in acetonitrile yields a different type of compound with the composition (Et4N)2[Fe4(SCH2Ph)6Br4] (5): triclinic; P1; a = 12.874 (2) A, b = 13.949 (3) A, c = 43.272 (8) A, α= 94.85 (2)°, β = 94.07 (2)°, y = 114.98 (1)°; 4; 7402; 6.7. Anions of type 4 contain a new type of inorganic ring system, Fe3(jU2-SR)3, distinguished by its nearly or exactly planar conformation and unusually large Fe-S-Fe angles of ~ 140°, the largest values observed for this angle type. The phenyl groups in 4a and 4c (which has imposed C3h symmetry) are coplanar with the Fe3S3 rings. In 4d these groups are rotated by ~90° owing to the steric effects of the methyl groups. Distorted tetrahedral coordination at each Fe(II) atom is completed by two halide ligands. The planar structure is to be contrasted with the chair-type conformation of the rings in 5 and other species with adamantane-like stereochemistry. Ring conformations influence the extent of antiferromagnetic interactions in the solid state. Magnetic susceptibility data of planar 4a are fit well by two -J values at 19–22 cm-1. For [Fe4(SR)10]2_, containing adamantane-like Fe4(M2-SR)6 cages with Fe3(ji2-SR)3 rings in the chair conformation, best fits of the magnetic data give one -J value at 32 or 36 cm-1 and one or two others at 42–66 cm-1 depending on the coupling symmetry. MO calculations at the extended Huckel level on suggest that the inherently stable conformation of the ring is planar. This structure is set in large measure by the stereochemical preference of the S-Fe-S angle and by transannular nonbonded repulsions. The decrease in Cl—Cl distances from 6.6 to 3.9 A in changing from the planar to the chair conformation emphasizes the latter effect. In acetonitrile the equilibrium has been approached in both directions and is shifted to the left with decreasing temperature. At K the formation of is essentially complete. In this temperature range the magnetic moment per Fe atom is 4.1-4.2 for 4a in solution and as its crystalline Et4N+ salt. This excellent agreement establishes a planar ring conformation in solution. Thus the results of three crystal structures, MO calculations, and solution and solid state magnetic measurements collectively demonstrate that the near or exact planarity of the Fe3S3 cycles in is an intrinsic property.