Synthesis of hydride and alkyl compounds containing the Cp*Os(NO) fragment.: Crystal structure of [Cp*Os(μ-NO)]2

Treatment of the pentamethylcyclopentadienyl (Cp*) compound Cp*Os(NO)Br-2 (1) with NaBH4 yields the dihydride Cp*Os(NO)Hz. The dihydride loses Hz over several days in solution to form [Cp*Os(mu-NO)](2); this dinuclear compound can also be formed directly by reduction of 1 with zinc powder. The X-ray crystal structure of [Cp*Os(mu-NO)](2) shows that nitrosyl ligands are bridging and that the Os-Os distance is 2.539(1) Angstrom. Compound 1 reacts with MgR2 to form the monoalkylated products Cp*Os(NO)RBr, where R = Me, CH2SiMe3, Ph, or o-Tol. The slow ligand substitution kinetics of this reaction prevent the formation of dialkyls of stoichiometry Cp*Os(NO)R-2, even after long reaction times or when a large excess of alkylating agent is used. Dialkyls such as Cp*Os(NO)Me-2 can, however, be obtained by treatment of Cp*Os(NO)MeBr with silver trifluoromethanesulfonate followed by addition of dimethylmagnesium.