CONVERSION OF THE CLUSTERS [OS-3(L,3-DIYNE)(CO)(10)] INTO BIS(ALKYNYL) CLUSTERS BY CARBON-CARBON BOND-CLEAVAGE

The symmetrical 1,3-diynes, RC(2)C(2)R (R=Me, Et, Ph, Bu(t), SiMe(3),) , react with [Os-3,(CO)(10)(MeCN)(2)] to give the clusters [Os-3(mu(3) ,eta(2)-RC(2)C(2)R)(mu-CO)(CO)(9)]. The mu(3),eta(2)-coordinated alkyn e group does not exchange with the non-coordinated one. As a consequen ce the unsymmetrical diyne, PhC(2)C(2)SiMe(3), gives non-interconverti ng and separable isomers of [Os-3(mu(3) eta(2)-PhC(2)C(2)SiMe(3))(mu-C O)(CO)(9)] which differ only in which of the two alkyne groups is coor dinated. Thermal decarbonylations of the compounds containing RC(2)C(2 )R (R=Ph, Bu(t) or SiMe(3)) and the isomers containing the unsymmetric al diyne, PhC(2)C(2)SiMe(3), lead by carbon-carbon bond cleavage to bi s(alkynyl) dusters of the type [Os-3(mu(2) eta(1)-C(2)R(2))(mu(3),eta( 2)-C(2)R(2))(CO)(9)] where R(1)=R(2)=Ph, Bu(t) SiMe(3) and only one is omer from the mixed diyne with R(1)=Ph and R(2)=SiMe(3). XRD studies o n this isomer established that C(2)SiMe(3) is in the triply-bridging p osition and C(2)Ph in the doubly-bridging position. Although the C(2)P h ligand is only bonded through the alpha-carbon atom, we consider it to be alpha, pi-bonded and a three-electron donor. It spans two osmium atoms that are not bonded (Os...Os=3.297(2) Angstrom). Thermolysis of the EtC(2)C(2)Et complex leads to carbon-hydrogen rather than carbon- carbon cleavage to give the allenyl cluster Os3H(mu(3),eta(2),eta(2)-M eCH=C=C-C(2)Et)(CO)(9)], while the MeC(2)C(2)Me cluster decarbonylates to [Os-3(C(4)Me(2))(CO)(9)] of unknown structure but which may be [Os -3(CMe)-(CC(2)Me)(CO)(9)].