Reactions of [MO4](2-) (M = Mo, W) with certain carbon and silicon electrophiles were investigated in acetonitrile in order to produce species of potential utility in the synthesis of analogues of the sites in the xanthine oxidoreductase enzyme family. Silylation of [MoO4](2-) affords [MoO3(OSiPh3)(1-), which with Ph3SiSH is converted to [MoO2S(OSiPh3)(1-), Reaction with (Ph3C)(PF6)/HS- yields the tetrahedral monosulfido species [MO3S](2-), previously obtained only from the aqueous system [MO4](2-)/H2S. Dithiolene chelate rings are readily introduced upon reaction with 1,2-C6H4-(SSiMe3)(2,) leading to the square pyramidal trioxo complexes [MO3(bdt)](2-), a previously unknown dithiolene molecular type. Further ring insertion occurs upon reaction of [WO3(bdt)](2-) with 1,2-C6H4(SSiMe3)(2), giving [WO2(bdt)(2)](2-). Related reactions occur with [ReO4](l-). Treatment with 1 equiv of (Me3Si)(2)S produces [ReO3S](1-); with 3 equiv of 1,2-C6H4(SSiMe3)(2), [ReO(bdt)(2)](1-) is obtained with concomitant Re-VII --> Re-V reduction. X-ray structures are reported for [MO3S](Z-) (M = Mo, W, z = 2; M = Re, z = 1), [MO3(bdt)](2-), and [WO2(OSiPh3)(bdt)](1-), a silylation product of [WO3(bdt)](2-). [MoO3(bdt)](2-) is related to the site of inactive sulfite oxicase, and [WO2(OSiPh3)(bdt)]1- should closely approximate the metric features of the [(dithiolene)MoO2(OH)] site in inactive aldehyde/xanthine oxidoreductase. This work provides convenient syntheses of known and new derivatives of tetraoxometalates, among which is entry to a unique class of oxo-monodithiolene complexes.