ALKYNE-VINYLIDENE COUPLING IN THE REACTIONS OF THE ALKYNE COMPLEX OS-3(MU-CO)(CO)(9)(MU(3)-ME3SIC2ME) WITH ME3SIC=CR (R = ME, BU-II) - THE STRUCTURE AND STEREODYNAMIC BEHAVIOR OF CLUSTERS OS-3(CO)(9)(MU(3)-C(SIME3)=C(ME)C=C(SIME3)R) CONTAINING AN OSMACYCLOBUTENE MOIETY

Reactions of the alkyne cluster Os-3(mu-CO)(CO)(9)(mu(3)-Me3SiC2Me) wi th alkynes Me3SiC=CR (R = Me, Bu-n) in refluxing hexane result in the formation of clusters Os-3(CO)(9){mu(3)-C(SiMe3)=C(Me)C=C(SiMe3)R} (2a : R = Me; 3a: R = Bu-n). The dienediyl ligand in these complexes is fo rmed by alkyne-vinylidene coupling, with vinylidene generated in the c ourse of reaction from the alkyne molecule by the acetylene-vinylidene rearrangement involving a 1,2-shift of the Me3Si group. The structure of cluster 3a was determined by X-ray structural analysis. The diened iyl ligand is coordinated to three metal atoms of the cluster framewor k by two pi-ethylene bonds with two osmium atoms and two sigma-bonds w ith the third osmium atom with the formation of the osmacyclobutene mo iety. The H-1 and C-13 NMR study of (CO)-C-13-enriched samples of clus ters 2a and 3a revealed the stereochemical nonrigidity of these molecu les due to the exchange of the hydrocarbon and carbonyl ligands.