The cis-dioxo-molybdenum(VI) complexes, [MoO2(L-H)(2)](2-) (1b), [MoO2(Ls)(2)](2-) (2b), and [MoO2(L-O)(2)](2-) (3b) (L-H = cyclohexene-1,2-dithiolate, Ls = 2,3-dihydro-2H-thiopyran-4,5-dithiolate, and L-O = 2,3-dihydro-2H-pyran-4,5-dithiolate), with new aliphatic dithiolene ligands were prepared and investigated by infrared (IR) and UV-vis spectroscopic and electrochemical methods. The mono-oxo-molybdenum(IV) complexes, [MoO(L-H)(2)](2-) (1a), [MoO(L-S)(2)](2-) (2a), and [MoO(L-O)(2)](2-) (3a), were further characterized by X-ray crystal structural determinations. The IR and resonance Raman spectroscopic studies suggested that these cis-dioxo molybdenum(VI) complexes (1b-3b) had weaker Mo=O bonds than the common (MoO2)-O-VI complexes, Complexes 1b-3b also exhibited strong absorption bands in the visible regions assigned as charge-transfer bands from the dithiolene ligands to the CiS-MoO2 cores. Because the oxygen atoms of the CiS-(MOO2)-O-VI cores are relatively nucleophilic, these complexes were unstable in protic solvents and protonation might occur to produce (MoO)-O-VI(OH), as observed with the oxidized state of arsenite oxidase.