The existence of a universal pterin dithiolene cofactor ligand for the molybdenum and tungsten oxotransferases supports a biological significance of the fundamental chemistry of mono-and bis(dithiolene) complexes of these elements. Members of the dimethyl sulfoxide (DMSO) reductase family of enzymes contain two pterin dithiolene ligands: at least one enzyme functions by using the minimal reaction couple Mo-IV + Me2SO <--(-->) (MoO)-O-VI + Me2S. Accordingly, the synthesis, structures, and reactivity of bis(dithiolene)Mo(IV,VI) complexes of benzene-1,2-dithiolate and related Ligands have been investigated. A convenient synthesis of square pyramidal [(MoO)-O-IV(S2C2R2)(2)](2-) complexes is reported. Compounds of the type [Mo(S2C2R2)(2)(R＇NC)(2)], including [Mo(bdt)(2)(MeNC)(2)] (13), were prepared by reacting Na-2(S2C2R2) and R＇NC with [MoCl4(MeCN)(2)] and were shown to be identical (R＇ = Me) to byproducts in the synthesis of [(MoO)-O-IV(S2C2R2)(2)](2-). In one such reaction, the Fischer carbene complex [Mo(Me(4)bdt)(2)(MeNC)(CMe(4)bdt)] (16) was isolated. Silylation of [(MoO)-O-IV(bdt)(2)](2-) affords [Mo-IV(bdt)(2)((OSiBuPh2)-Ph-t)](-) (8); an analogous reaction of [MoO2(bdt)(2)](2-) yields [(MoO)-O-VI(bdt)(2)((OSiBuPh2)-Ph-t)](-) (11). The structures of square pyramidal 8 and cis-octahedral 11 reveal them to be minimal unconstrained representations of the des-oxo [Mo-IV(S(2)pd)(2)(O . Ser)] and mono-ore [(MoO)-O-VI(S(2)pd)(2)(O . Ser)] sites, respectively, of Rhodobacter sphaeroides DMSO reductase. This description applies in the limit of symmetrical dithiolene coordination; silyloxide is a simulator of serinate binding. Complex 8 shows limited reactivity with sulfoxides, and 11 is unreactive toward sulfides. However, 11 is reduced to 8 by tertiary phosphines; with excess phosphine, [Mo(bdt)(2)(PMePh2)(2)] (17) was formed. This compound was also prepared independently from [MoCl4(MeCN)(2)] and the phosphine. The compounds 13, 16, and 17 form an isoelectronic set with idealized trigonal prismatic (C-2v) stereochemistry. These results complement a parallel development of bis(dithiolene)W(IV,VI) complexes as active-site analogues of tungstoenzymes (Lorber, C.; Donahue, J. P.; Goddard, C. G,; Nordlander, E.; Helm, R. H, J. Aln. Chern. Sec. 1998, 120, 8102). Certain comparisons between the properties of molybdenum and tungsten bis(dithiolenes) are offered. (bdt = benzene-1,2-dithiolate(2-), S(2)pd = pterin dithiolene(2-).).