The cofactor (M-center) of the MoFe protein of nitrogenase, a MoFe7S9: homocitrate cluster, contains six Fe sites with a (distorted) trigonal sulfido coordination. These sites exhibit unusually small quadrupole splittings, DeltaE(Q) approximate to 0.7 mm/s, and isomer shifts, sigma approximate to 0.41 mm/s. Mossbauer and ENDOR studies have provided the magnetic hyperfine tensors of all iron sites in the S = (3)/(2) state M-N, To assess the intrinsic zero-field splittings and hyperfine parameters of the cofactor sites, we have studied with Mossbauer spectroscopy two salts of the three-coordinated Fell thiolate complex [Fe(SR)(3)](-) (R = C6H2-2,4,6-tBu(3)) One of the salts, [Ph4P][Fe(SR)(3)].2MeCN.C7H8, 1, has a planar geometry with idealized C-3h symmetry. This S = 2 complex has an axial zero-field splitting with D = +10.2 cm(-1). The magnetic hyperfine tensor components A(x) = A(y) = -7.5 MHz and A(z) = -29.5 MHz reflect an orbital ground state with d(z)(2) symmetry. A(iso) = (A(x) +A(y) +A(z))/3 = -14.9 MHz, which includes the contact interaction (kappaP = -21.9 MHz) and an orbital contribution (+7 MHz), which is substantially smaller than A(iso) approximate to -22 MHz of the tetrahedral Fe-11(S-R)(4) sites of both rubredoxin and [PPh4](2)[Fe-11(SPh)(4)]. The largest component of the electric field gradient (EFG) tensor is negative, as expected for a d(z)(2) orbital. However, DeltaE(Q) = -0.83 mm/s, which is smaller than expected for a high-spin ferrous site. This reduction can be attributed to a ligand contribution, which in planar complexes provides a large positive EFG component perpendicular to the ligand plane. The isomer shift of 1, delta = 0.56 mm/s, approaches the delta-values reported for the six trigonal cofactor sites. The parameters of 1 and their importance for the cofactor cluster of nitrogenase are discussed.