The complexes, [Bu(4)N](2)[(MoO2)-O-VI(mnt)(2)] (1), [Bu(4)N](2)[(MoO)-O-IV(mnt)(2)] (2), and [Ph(3)PNPPh(3)][Et(4)N][(MoOCl)-O-V(mnt)(2)] (3) (mnt(2-) = 1,2-dicyanoethylenedithiolate) have been synthesized as possible models for active sites of sulfite oxidase which is proposed to contain molybdenum cofactor with dithiolene coordination around molybdenum. The structure of the [Bu(4)P](+)l salt of complex anion of 1 has been determined by X-ray crystallography. The compound crystallizes in space group P2(1)/c, with a = 14.200(3) Angstrom, b = 19.402(4) Angstrom, c = 18.967(3) Angstrom, beta = 95.48(1)degrees, and Z = 4. [(MoO2)-O-VI(mnt)(2)](2-) is a distorted octahedron with the oxo groups cis to each other and trans to the dithiolene sulfur atoms. The complexes 1-3 have been characterized by IR, UV-visible, C-13 NMR, and negative ion FAB mass spectra. Complex 1 shows a quasireversible reduction and proton coupled electron transfer reaction. Complex 2 undergoes an one-electron reversible oxidation; but on the coulometric time scale it disproportionates to a tris dithiolene complex, [Mo-IV(mnt)(3)](2-) and MoO3. Complex 2 in the presence of Cl- is oxidized irreversibly with the appearance of a new quasireversible couple corresponding to the electrochemical detection of [(MoOCl)-O-V(mnt)(2)](2-)/[(MoOCl)-O-IV(mnt)2](3-). The EPR parameters of 3 and [(MoO)-O-V(mnt)(2)](1-) are reported. The Cl-35,Cl-37 superhyperfine splitting of the chloro complex 3 is shown in relevance to Mo-Cl interaction in native sulfite oxidase. Complex 1 oxidizes HSO3- to HSO4- with the formation of 2 and without forming the biologically irrelevant mu-oxo Mo(V) dimer. This reaction follows enzymatic substrate saturation kinetics with apparent K-M (Michaelis-Menten constant) = 0.010(+/-0.001) M and k(2) (k(Obs) at substrate saturation concentration and is proportional to V-max) = 0.87(+/-0.04) s(-1) in MeCN/H2O(1:1) medium at 20 degrees C.