Carbonyloxotungsten(IV) complexes, Tp*WOX(CO), are produced in the reactions of dioxygen (for X = Cl, Br, I) or pyridine N-oxide [for X = S2P(OPr')(2), S2PPh2] with Tp*WX(CO)(2) [Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)-borate]. Analogous carbonylsulfidotungsten(IV) species, Tp*WSX(CO), result from the reactions of Tp*WX(kappa(2)-MeCN)-(CO) with propylene sulfide. The carbonyloxo complexes exhibit nu(CO) and nu(W=O) IR bands in the 1995-1965 and 957-951 cm(-1) regions, respectively; the nu(CO) and nu(W=S) bands of the carbonylsulfido species appear at 1970-1937 and 512-502 cm(-1), respectively. The complexes possess C-1 symmetry and display carbonyl C-13 NMR resonances at delta 272-287, with J(WC) 160-196 Hz. The crystal structures of Tp*WO(S2PPh2)(CO) and Tp*WS(S-2-PPh2)(CO)(.)0.5CHCl(3) reveal distorted octahedral tungsten centers coordinated by a fac tridentate Tp* ligand and mutually cis, monodentate chalcogenido [d(W=O) = 1.698(4) angstrom; d(W=S) = 2.135(4) angstrom], carbonyl, and dithiophosphinato ligands. In refluxing toluene, Tp*WOI(CO) converts into purple, mixed-valence Tp*(WI)-I-III(CO)(mu-O)W(V)OITp*. The dinuclear complex contains a nearly linear [173.1(6)degrees] mu-oxo bridge connecting disparate distorted octahedral tungsten centers. The metrical parameters and spectroscopic properties are consistent with the presence of a W-III/W-V mixed-valence species, possessing a filled, delocalized three-center (W-O-W) pi bond and a localized (on W-III), filled d(pi) orbital that back-bonds to the carbonyl ligand.