The syntheses, and spectroscopic, structural and electrochemical properties of cis-dioxotungsten(vl) complexes LWO2X [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate; X = Cl-, NCS-, OMe-, O2CH-, OPh-, SPh-, S2PPh2-S-, SePh-] are described. Reaction of LWO2Cl with group 1 salts (MX)-X-I and 18-crown-6 in refluxing toluene was employed in the syntheses of derivatives with X = NCS-, OPh-, SPh-, and SePh-, while reaction of LWO2(SePh) with methanol and LWO2(SPh) with formic acid yielded LWO2(OMe) and LWO2(O2CH), respectively. The complex LWO2(S2PPh2-S) was prepared by reacting LW(S2PPh2-S)(CO)(2) with pyridine N-oxide. The complexes exhibit two nu(WO2) infrared bands, at 935-960 cm(-1) and 900-915 cm(-1), and H-1 NMR spectra consistent with C-5, symmetry. Orange crystals of cis-LWO2(SePh) are monoclinic, space group P2(1)/c, with a = 18.385(6) Angstrom, b = 8.102(1) Angstrom, c = 18.284(5) Angstrom, beta = 117.15(2)degrees, V = 2423(2) Angstrom(3), and Z = 4. The structure was solved by direct methods and refined to R = 0.035 (R-W = 0.033) for 3668 reflections with I greater than or equal to 3.0 sigma(I). The. mononuclear complex exhibits a distorted octahedral coordination sphere composed of a selenophenolate ligand [W-Se = 2.535(1) Angstrom], two terminal oxo groups [W=O = 1.716(4) and 1.721(4) Angstrom], and a facially tridentate L ligand. In acetonitrile, the complexes undergo a one-electron reduction at very cathodic potentials (E-1/2 = -1.71 to E-pc = -1.05 V vs SCE), some 560-620 mV more negative than observed for analogous molybdenum complexes (Inorg. Chem. 1996, 35, 7508). The complexes are very stable and do not participate in clean oxygen atom transfer or coupled electron-proton transfer reactions.