Structural analogues of the reduced (Mo(TV)) sites of members of the DMSO reductase family of molybdoenzymes are sought. These sites usually contain two pterin--dithiolene cofactor ligands and one protein-based ligand. Reaction of [Mo(MeCN)(3)(CO)(3)] and [Ni(S2C2R2)(2)] affords the trigonal prismatic complexes [MO(CO)(2)(S2C2R2)(2)] (R = Me (1), Ph (2)), which by carbonyl substitution serve as useful precursors to a variety of bis(dithiolene)molybdenum(IV,V) complexes. Reaction of 1 with Et4NOH yields [MoO(S2C2Me2)(2)](2-) (3), which is readily oxidized to [MoO(S2C2Me2)(2)](1-) (4). The hindered arene oxide ligands ArO- afford the square pyramidal complexes [Mo(OAr)(S2C2R2)(2)](1-) (5, 6). The ligands PhQ(-) afford the trigonal prismatic monocarbonyls [Mo(CO)(QPh)(S2C2Me2)(2)](1-)(Q = S (8), Se (12)) while the bulky ligand ArS- forms square pyramidal [Mo(SAr)(S2C2R2)(2)](1-) (9, 10). In contrast, reactions with ArSe- result in [Mo(CO)(SeAr)(S2C2R2)(2)](1-) (14, 15), which have not been successfully decarbonylated. Other compounds prepared by substitution reactions of 1 and 2 include the bridged dimers [Mo-2- (mu-Q)(2)(S2C2Me2)(4)](2-) (Q = S (7), Se (11)) and [Mo-2(mu-SePh)(2)(S2C2Ph2)(4)](2-) (13) The complexes 1, 3-5, 7-10, 12-14, [Mo(S2C2Me2)(3)] (16), and [Mo(S2C2Me2)(3)](1-) (17) were characterized by X-ray structure determinations. Certain complexes approach the binding arrangements in at least one DMSO reductase (5/6) and its Ser/Cys mutant, and in dissimilatory nitrate reductases (9/10). This investigation provides the initial demonstration of the new types of bis(dithiolene)molybdenum(IV) complexes available through [Mo(CO)(2)(S2C2R2)(2)] precursors, some of which will be utilized in reactivity studies. (Ar = 2,6-diisopropylphenyl or 2,4,6-triisopropylphenyl.).