The new bis(dithiolate) complexes Mo(1,2-S2-4-R-C6H3)2(PMePh2)2, R = H, Me, are readily synthesized from three different types of starting materials:trans-Mo(N2)2(PMePh2)4, MoCl4(PMePh2)2, and MoOCl2(PMePh2)3. The31P NMR spectra of the dithiolate complexes at low temperature are consistent with acisstereochemistry of the PMePh2ligands and isomerism with respect to inversion at sulfur and relative positioning of the methyl groups on the chelating 1,2-S2-4-Me-C6H3ligands. The singlet in the31P NMR spectrum at room temperature arises from rapid interconversion of these isomers. Substitution of one or two of the PMePh2ligands in the dithiolate complexes proceeds under mild conditions to give the complexes Mo(S2C6H4)2(CO)(PMePh2), Mo(S2C6H4)2(CN-t-Bu)(PMePh2), Mo(S2C6H3Me)2(dppe), Mo(S2C6H4)2(depe), Mo(S2C6H4)2(PMePh2)(PMe3), Mo(S2C6H4)2(PMePh2)(P(OMe)3), and Mo(S2C6H4)2(P(OMe)3)2. The31P and13C NMR spectroscopy of these bis(dithiolate) complexes suggest that they have monodentate ligandscisdisposed.A reversible equilibrium exists between Mo(S2C6H4)2(PMePh2)2and Mo(S2C6H4)2(CO)(PMePh2) in the presence of carbon monoxide in solution. Isolation of Mo(S2C6H4)2(CO)(PMePh2) is facilitated by use of CH3I as a "phosphine sponge" reagent. If the sulfur ligands are viewed as dianions, then this is the first thermally stable carbonyl complex of molybdenum(IV).Keywords: molybdenum, dithiolate, diphenylmethylphosphine, carbonyl, NMR.