The active sites of the xanthine oxidase and sulfite oxidase enzyme families contain one pterin-dithiolene cofactor ligand bound to a molybdenum atom. Consequently, monodithiolene molybdenum complexes have been sought by exploratory synthesis for structural and reactivity studies. Reaction of [MoO(S2C2Me2)(2)](1-) or [MoO(bdt)(2)](1-) with PhSeCl results in removal of one dithiolate ligand and formation of [MoOCl2(S2C2Me2)(1-) (1) or [MoOCl2(bdt)](1-) (2), which undergoes ligand substitution reactions to form other monodithiolene complexes [MoO(2-AdS)(2)(S2C2Me2)](1-) (3), [MoO(SR)(2)(bdt)](1-) (R = 2-Ad (4), 2,4,6-(Pr3C6H2)-C-i (5)), and [MoOCl(SC6H2-2,4,6-Pr-3(i))(bdt)](1-) (6) (Ad = 2-adamantyl, bdt = benzene-1,2-dithiolate). These complexes have square pyramidal structures with apical oxo ligands, exhibit rhombic EPR spectra, and 3-5 are electrochemically reducible to (MoO)-O-IV species, Complexes 1-6 constitute the first examples of five-coordinate monodithiolene (MoO)-O-V complexes; 6 approaches the proposed structure of the high-pH form of sulfite Oxidase. Treatment Of [MoO2(OSiPh3)(2)] with Li-2(bdt) in THF affords [MoO2(OSiPh3)(bdt)](1-) (8). Reaction of 8 with 2,4,6-(Pr3C6H2SH)-C-i in acetonitrile gives [MoO2(SC6H2-2,4,6-Pr-3(i))(bdt)](1-) (9, 55%). Complexes 8 and 9 are square pyramidal with apical and basal oxo ligands. With one dithiolene and one thiolate ligand of a square pyramidal (MoO2S3)-O-VI coordination unit, 9 closely resembles the oxidized sites in sulfite oxidase and assimilatory nitrate reductase as deduced from crystallography (sulfite oxidase) and Mo EXAFS. The complex is the first structural analogue of the active sites in fully oxidized members of the sulfite oxidase family. This work provides a starting point for the development of both structural and reactivity analogues of members of this family.