A series of dioxo-Mo(VI) Schiff base complexes of the type Mo02(R-L)(solv) has been prepared: L = 2-(salicylideneamino)-phenolate(2-) (sap) or 2-(salicylideneamino)benzenethiolato(2-) (ssp); solv = solvent. Salicylidene ring substituents R were selected to promote solubility and crystallinity. The unsubstituted complexes MoO2(sap)(solv) (1) and Mo02(ssp)(solv) (3) have been previously reported. The structures of two members of the series were determined. [MoO2(5-t-Busap)(MeOH)]-MeOH (2) crystallizes in space group P21with a = 6.874 (2) Á,b = 11.333 (2) A, c = 13.295 (4) A, β= 98.79 (2)°, and Z = 2 and was refined to R = 2.9%. (Me4N)[Mo02(5-S03ssp)] (4) was obtained in space group P21/c with a = 11.156 (4) Á,b= 12.747 (7) A, c = 14.055 (2) A, = 97.06°, and Z = 4 and was refined to R = 3.8%. The two complexes contain the cis Mo02group with O-Mo-O angles of 105°. Other structural features include severely distorted octahedral coordination, mer disposition of the tridentate ligands, and a weakly bound solv molecule trans to an oxo ligand (2); these attributes apply to Mo(V,IV) complexes as well. Complex 4 is a chain polymer linked by coordination of the sulfonate group trans to an oxo ligand. Reaction of Mo02(R-L)(solv) with Ph2MeP in DMF affords products with UV-visible spectra previously attributed to the Mo(IV) complexes MoOL(solv). These products have been identified as μ-oxo binuclear Mo(V) complexes by structure determinations. Mo2O3-(3-t-Bussp)2(DMF)2(11) crystallizes in space group Pbca with a = 9.245 (3) A, b = 20.763 (6) A, c = 22.417 (7) A, and Z = 4, while [Mo2O3(3-EtOssp)2(DMF)2]-2DMF (12) was found in space group P1 with a = 8.347 (1) A, b = 12.110 (2) A, c = 12.194 (2) A, a = 100.73 (1)°, β = 93.81 (1)°, ϒ= 103.27 (1)°, and Z = 1. The structures were refined to 4.1% (11) and 2.6% (12). Both complexes have imposed centrosymmetry. When the reduction reaction with phosphine was carried out in the presence of 2,2/-bipyridyl, the Mo(IV) products MoO(R-L)(bpy) (R = H (13), 3-t-Bu (14)) were isolated. Compound 14 crystallizes in space group C2/c with a = 36.348 (5) A, b = 10.256 (1) A, c = 16.102 (2) A, β= 97.44 (1)°, and Z = 8; the structure was refined to R = 8.8%. Compound 13 was correctly formulated in an earlier report. Trapping of the MoIV0 state with bpy demonstrates that it is the instantaneous product of reductive atom transfer from the MoVIO2oxidation level. In the absence of a trapping reagent, Mo(V) complexes such as 11 and 12 are formed in the rapid, irreversible reaction MoO2(R-L)(solv) + MoO(R-L)(solv) → Mo2O3(R-L)2(solv)2. Criteria for the formation of Mo2O3complexes in solution include demonstration of the reaction stoichiometry 2MoO2(R-L)(solv) + R3P Mo2O3(R-L)2(solv)2+ R3PO by31P NMR, detection of diastereomers, and characteristic absorption spectra. An additional criterion follows from proof of the reaction stoichiometry Mo203(R-L)3(solv)2+ (RF)2SO →2MoO2(R-L)(solv) + (RF)2S by19F NMR (RF= p-C6H4F). Because in a variety of solvents no Mo(V) complex undergoes detectable dissociation, it is probable that the foregoing reaction involves the intact Mo203and proceeds by coordination of sulfoxide to the Mo followed by intramolecular atom and electron transfer. The results of this work emphasize the pervasiveness of formation of Mo203species in oxo transfer reaction systems.