COMPARATIVE REACTIVITY OF ISOELECTRONIC NITRIDO, METHYLIMIDO, AND OXO COMPLEXES OF OSMIUM(VI)

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 os(O)(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(O)(CH2SiMe3)4 are reductively carbonylated under CO pressure to produce Os(CO)5, (Me3SiCH2)2CO, and either MeNCO or CO2. The methylimido complex is much more reactive than the oxo complex, and the acyl species Os(NMe)(C(O)CH2SiMe3)(CH2SiMe3)3 and Os(NMe)(C(O)-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 tert-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. © 1990, American Chemical Society. All rights reserved.