Kinetics of the oxygen atom transfer reactions [M-IV(QC(6)H(2)-2,4,6-Pr-3(i))(S2C2Me2)(2)](1-) + XO --> [(MO)-O-VI(QC(6)H(2)-2,4,6Pr(3)(i))(S2C2Me2)(2)](1-) + X in acetonitrile with substrates XO = NO3- and (CH2)(4)SO have been determined. The reactants are bis(dithiolene) complexes with M = Mo, W and sterically encumbered axial ligands with Q = O, S to stabilize mononuclear square pyramidal structures. The complex [MOIV(SC6H2-2,4,6-Pr-3(i))(S2C2Me2)(2)](1-) is an analogue of the active site of dissimilatory nitrate reductase which in the reduced state contains a molybdenum atom bound by two pyranopterindithiolene ligands and a cysteinate residue. Nitrate reduction was studied with tungsten complexes because of unfavorable stability properties of the molybdenum complexes. Product nitrite was detected by a colorimetric method. All reactions with both substrates are second-order with associative transition states (DeltaS(double dagger) similar to -20 eu). Variation of atoms M and Q, together with data from prior work, allows certain kinetics comparisons to be made. Among them, k(2)(W)/k(2)(Mo) = 25 for (CH2)(4)SO reduction (Q = S), an expression of the kinetic metal effect. Further, k(2)(S)/k(2)(O) = 28 and similar to10(4) for nitrate and (CH2)(4)SO reduction, respectively, effects attributed to relatively more steric congestion in achieving the transition state with hindered phenolate vs thiolate ligands. The effect is more pronounced with the larger substrate. These results demonstrate the feasibility of tungsten-mediated nitrate reduction by direct atom transfer using molecules with both axial thiolate and phenolate ligands. Complexes of the type [M-IV(OR)(S2C2Me2)(2)] are capable of reducing biological N-oxide, S-oxide, and nitrate substrates and thus constitute functional analogue reaction systems of enzymic transformations.