SYNTHESIS, CHARACTERIZATION AND REACTIVITY OF ALKYL, ALKYLIDENE AND ALKYLIDYNE DERIVATIVES OF TRIOSMIUMDODECACARBONYL

The methylidyne (CH) ligand, a potential intermediate in the hydrogenolysis of carbon monoxide, is an object of current interest. HOs(,3)(CO)(,10)(CH), a new complex with a triply bridging methylidyne has been prepared. In contrast with previous examples the methylidyne ligand in this complex interacts much more strongly with two of the metal centers than the third. The structure of this methylidyne complex has been established by X-ray cyrstallography. ('13)C NMR data for HOs(,3)(CO)(,10)(CH) in solution are fully consistent with the solid state structure. Bonding in the methylidyne complex may be regarded as two-electron donation from the formally saturated Os(CO)(,4) center to the electrophilic carbon atom of the bridging CH moiety. The facile reactions of HOs(,3)(CO)(,10)(CH) with various nucleophiles provide further evidence for the electrophilic nature of this methylidyne ligand.; Bridging alkyl derivatives of transition metal clusters are rare, yet their properties are potentially relevant to an understanding of Fischer-Tropsch and related processes involving C-H bond activation. We have prepared a series of triosmium hydrido alkyls containing both (alpha) and (beta) hydrogens by a similar synthetic procedure. Nucleophilic attack of LiHBEt(,3) at the (beta) carbon of the triosmium alkenyl complexes, HOs(,3)(CO)(,10)(CH=CHR) R = H, CH(,3) or attack of CH(,2)=PPh(,3) at HOs(,3)(CO) (,10)(CH=CH(,2)) yields anionic and zwitterionic alkylidenes respectively. The alkylidenes are converted to alkyls by protonation. The alkyls contain (alpha)-C-H-Os three-center two-electron bonds. ('13)C NMR spectra of the ethyl complex, HOs(,3)(CO)(,10)(CH(,2)CH(,3)), indicate the diasterotopic methylene hydrogens are equilibrated via a symmetric unsaturated intermediate, electronically analogous to H(,2)Os(,3)(CO)(,10). The alkyl ligands are subject to both (alpha) and (beta) elimination processes, the latter of which predominates at higher temperature. The addition of nucleophiles induces reductive elimination of the alkane.