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The structures, physical properties, and reaction chemistry of isoelectronic alkyl complexes of osmium(Vl) are compared. The three, five-coordinate alkyl complexes [NBun4][Os(N)(CH2SiMe3)4], Os(NMe)(CH2SiMe3)4, and 0s(0)(CH2SiMe3)4 have been characterized by single-crystal X-ray diffraction, NMR, IR, UV-visible spectroscopy, and by electrochemical techniques. All have a square-pyramidal structure in the solid state, with the imido and oxo complexes having a slight distortion toward trigonal bipyramidal. The terminal heteroatom ligand is triply bonded to the metal in each compound, but the extent of donation to the metal decreases in the order Os=N > Os^NMe > Os^O. The nitrido complex, [NBun4][Os(N)(CH2SiMe3)4],does not react with carbon monoxide, while both Os(NMe)(CH2SiMe3)4 and Os(0)(CH2SiMe3)4 are reductively carbonylated under CO pressure to produce Os(CO)5, (Me3SiCH2)2CO, and either MeNCO or C02. The methylimido complex is much more reactive than the oxo complex, and the acyl species 0s(NMe)(C(0)CH2SiMe3)(CH2SiMe3)3 and Os(NMe)(C(0)-CH2SiMe3)2(CH2SiMe3)2 are produced with 1 and 2 equiv of carbon monoxide, respectively. An insertion product, Os-(NMe)|C(NCMe3)CH2SiMe3)(CH2SiMe3)3, is formed in the reaction between Os(NMe)(CH2SiMe3)4 and feri-butylisonitrile. The order of reactivity toward electrophilic attack is [NBun4][Os(N)(CH2SiMe3)4] > Os(NMe)(CH2SiMe3)4 » Os(O)-(CH2SiMe3)4. The nitrido complex forms 1:1 adducts with Lewis acids, alkylates at nitrogen upon reaction with organic electrophiles, and reacts with mineral acids by cleavage of one or two of the metal-carbon bonds. Electrophilic cleavage of one metal-carbon bond occurs more slowly with the methylimido complex upon reaction with HBF4 or HC1. The oxo complex is stable toward electrophiles. The observed differences in reactivity can be ascribed to subtle changes in the bonding between the metal and the terminal heteroatom ligand and to the formation of a structural isomer of the methylimido complex.

second sulfur atom. Further oxidation to the sulfenic acid derivative and lactonization would then yield thiotropocin. Experimental SectionMethods and Materials. NMR spectra were obtained at 400.13 or 250.13 MHz ( ) and 100.6 or 62.9 MHz (,3C). 'H/13C long-range HETCOSY spectral acquisition parameters: 20 000-Hz sweep width in the F2 dimension; 128 spectra (860 scans each) accumulated with a 2.0-s relaxation delay. Resolution was 9.766 Hz per point in the F2 dimension and 13.378 Hz per point in the Fj dimension. [l,2-l3C2]Phenylacetic acid (99 atom %), DL-[3-l3C]phenylalanine (99 atom %) and d-[U-i3C6]glucose (90 atom %) were purchased from ISOTEC, Inc. Thiotropocin (1) was converted to the p-bromobenzyl thioether derivative 2 as previously described.111 Fermentation of Pseudomonas CB-104. A loopful of cells from a cultivated slant was inoculated into 50 mL of seed medium in a 250-mL flask. The seed medium contained fructose (10 g), polypepton (BBL) (5 g), meat extract (Wako Pure Chemical Industries) (5 g), and NaCl (5 g) (pH 7.0) per liter. The inoculated seed culture was incubated at 28 °C for 2 days with reciprocal shaking at 100 rpm. The inoculum (0.8 mL) was transferred to 50 mL of the same seed medium and incubated at 24 °C for 1 day with 20 mL/min of aeration. The resulting culture (2.5 mL) was then transferred to 100 mL of production medium containing fructose (5 g) (sterilized separately), polypepton (0.5 g), meat extract (0.5 g), NaCl (0.5 g), and Na2S203 (0.05 g) (pH 7.0). The production medium was incubated at 24 °C for 36 h with 44 mL/min of aeration and 100-rpm agitation.Isolation and Purification of Thiotropocin. Cells from the above