Two monooxomolybdenum(IV) complexes with dithiolene-like benzenedithiolate ligands, (NEt(4))(2)[(MoO)-O-IV(Ph(3)Si-bdt)(2)] (1) (Ph(3)Si-bdt 3-(triphenylsilyl)-1,2-benzenedithiolato) and (NEt(4))(2)[(MoO)-O-IV(Ph(3)Si-tdt)(2)] (2) (Ph(3)Si-tdt = 5-(triphenylsilyl)-3,4-toluenedithiolato were synthesized as models of reduced state molybdenum oxidoreductase. The MoOS4 cores of 1 and (NEt(4))2[(MoO)-O-IV(tdt)(2)] (4) (tdt= 3,4-toluenedithiolato) have a square pyramidal geometry similar to that of (NEt(4))(2)[(MoO)-O-IV(bdt)(2)] (3) (bdt = I,2-benzenedithiolato) (Boyde et al., 1986). The substituent effect of the triphenylsilyl group was detected in UV/vis spectra and in electrochemical properties. In the UV/vis spectra the band energy and extinction coefficients of 1 and (both with a Ph(3)Si- group) are displaced to lower energy and slightly stronger (about 2,000 M(-1) cm(-1)) than 3 and 4, respectively. The Mo(IV)/Mo(V) redox potential of P shows a negative shift (-0.03 V) compared with that of 3. These differences are due to the direct or indirect effect of the electron donation from the phenyl group pi orbital and the p pi orbital of sulfur atom. In contrast tb 1, 3, and 4, only the complex, 2; that has the most bulky ligand, shows a quasi-reversible couple at +0.52 V (vs SCE) assignable to the Mo(V)/Mo(VT) redox process. The bulkiness of the ligand clearly contributes to the stabilization of the Mo(V)/Mo(VI) redox process of 2. In the O atom transfer reaction of the monooxomolybdenum(IV) benzenedithiolate complex with Me(3)NO, the corresponding dioxomolybdenum(VI) benzenedithiolate complex and Me(3)N are formed quantitatively. This indicates the absence of (mu-oxo)dimolybdenum(V)complex formation in the present reaction controlled by the unique electronic properties and the strength of chelation of the dithiolato ligands.