$C_{60}$ triosmium derivatives, $Os_3(CO)_8L(\mu_3-\eta^2,\eta^2,\eta^2-C_{60})(L=NCMe(2)$, $NMe_3(3)$, $CH_2CH_2(4))$ have been prepared by decarbonylation of $Os_3(CO)_9(\mu_3-\eta^2,\eta^2,\eta^2-C_{60})(1)$ with $Me_3NO$ in the presence of corresponding ligands. $Os_3(CO)_{8-n}(CH_2CH_2)(PhCH_2NC)_n(\mu_3-\eta^2,\eta^2,\eta^2-C_{60})$ (n=1 (5), 2(6)) have also been prepared by decarbonylation of $Os_3(CO)_{9-n}(PhCH_2NC)_n(\mu_3-\eta^2,\eta^2,\eta^2-C_{60})$(n=1, 2) with $Me_3NO$ followed by the substitution of ethylene ligand. The structures of 5 and 6 have been determined by single crystal X-ray diffraction studies. The ethylene ligand in each compound is coordinated to an osmium center in an $\eta^2$-fashion and occupies an equatorial site. The fluxional processes of 4 and 5 have been examined by variable temperature $^1H$ and $^{13}C$ NMR studies. Both compounds exhibit olefin rotation and restricted 3-fold rotation. The olefin rotation of $4(\DeltaG_c^{\ddag}=44.1\pm2.1kJmol^{-1})$ is slower than that of 3, indicating that metal-to-olefin π-backdonation is enhanced by the electron-donating benzyl isocyanide ligand.
