Brown TpiPrMoO(SR)(CO) (TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate; R = Et, iPr, Ph, p-tol, Bz) are formed when TpiPrMoO(SR)(NCMe) react with CO gas in toluene. The carbonyloxomolybdenum(IV) complexes exhibit nu(CO) and nu(Mo=O) IR bands at ca. 2025 and 935 cm(-1), respectively, and NMR spectra indicative of C(1) symmetry, with delta(C)(CO) ca. 250. The crystal structure of TpiPrMoO(SiPr)(CO), the first for a mononuclear carbonyloxomolybdenum complex, revealed a distorted octahedral geometry, with d(Mo=O) = 1.683(3) A, d(Mo-C) = 2.043(5) A, and angle(O=Mo-C) = 90.87(16) degrees . The blue-green acetonitrile precursors are generated by reacting cis-TpiPrMoO2(SR) with PPh3; they are unstable, display a single nu(Mo=O) IR band at ca. 950 cm(-1), and exhibit NMR spectra consistent with C1 symmetry. Red-brown cis-TpiPrMoO2(SR) (R = as above and tBu) are formed by metathesis of TpiPrMoO2Cl and HSR/NEt3 in dichloromethane. The complexes exhibit strong nu(MoO2) IR bands at ca. 925 and 895 cm(-1), and NMR spectra indicative of Cs symmetry; the isopropyl, p-tolyl, and benzyl derivatives possess distorted octahedral geometries, with d(Mo=O)(av) = 1.698 A and angle(MoO(2))(av) = 103.5 degrees.