The heterometal clusters MFe(4)S(6)(PEt(3))(4)Cl [M = V (1), Mo (2)] contain a cuboidal Fe4S3 fragment that is bridged by three mu(2)-S atoms to the heterometal, forming a MFe(4)S(6) core structure of idealized C-3 nu symmetry. Three symmetry-related (equatorial) Fe atoms and the M atom have terminal phosphine ligands while the unique (axial) Fe atom is terminally coordinated by chloride. These clusters and others prepared in this work are of primary interest because their Fe-4(mu(3)-S)(3)(mu(2)-S)(3) portion structurally resembles a corresponding portion of the core of the FeMo-cofactor cluster of nitrogenase established by protein crystallography. Improved preparations of 1 and 2 are reported. Reaction of these clusters with NaSR in acetonitrile/THF yielded VFe4S6(PEt(3))(4)(SR) [R = Ph (3), Et (5)] and MoFe4S6(PEt(3))(4)(SR) [R Ph (4), Et (6)]. Compound 3 crystallizes in triclinic space group P ($) over bar 1 with a = 12.094(3) Angstrom, b = 13.651(3) Angstrom, c = 16.840(6) Angstrom, alpha = 112.19(2)degrees, beta = 91.30(2)degrees, gamma = 114.70(2)degrees, and Z = 2. Compound 4 crystallizes in monoclinic space group P2(1)/n with a = 20.796(6) Angstrom, b = 19.006(4) Angstrom, c = 26.076(7) Angstrom, beta = 108.64(2)degrees, and Z = 8. Both compounds exhibit the core structure of 1 and 2; thiolate is bound at the axial site to form tetrahedral FeS3(SR) units. Molybdenum clusters 2 and 4 exhibit dimensionally expanded cuboidal fragments compared to vanadium clusters land 3. Collective structural and Mossbauer and H-1 NMR spectroscopic results indicate that the additional valence electron in the molybdenum clusters is largely delocalized in the Fe4S3 cuboidal portion. Cluster 1 exhibits the first example of a spin state equilibrium (S = 0 reversible arrow S = 1) in any iron-sulfur cluster. Clusters 2 and 4 have S = 1/2 or spin-admired (S = 1/2, 3/2) ground states. All clusters are readily identified by their characteristic H-1 NMR spectra, in which the axial thiolate substituents exhibit contact shifts indicative of negative spin delocalization.