The thermal reaction of n BuOCH=CHPEt 2 with Os 3 (CO) 12 gives a new osmium triangular cluster, Os 3 (μ-H)(μ 3 - n BuOC=CHPEt 2 )(CO) 9 (1), which was isolated in 62% yield. The structure of 1 has been determined by X-ray diffraction (P1¯, a = 12.914(3), b = 13.984(4), c = 16.375(4) Å, α = 66.83(2), β = 77.82(2), γ = 77.97(2)°, Z = 4, d calc = 2.552 g/cm 3 , R = 0.0399 for 6636 reflections). The μ 3 - n BuOC=CHPEt 2 in 1 acts as a five-electron donor. It forms a σ-bond with one Os atom and coordinates the second and the third Os atoms of the cluster by its olefinic bond and the phosphorus atom, respectively. Both complex 1 and Os 3 (μ-H)(μ-CH=CHR)(CO) 10 (R= n C 6 H 13 , Ph) catalyze the silylation of terminal olefins with Et 3 SiH to yield the corresponding trans-triethylvinylsilanes and alkanes in moderate yield even at ambient temperature. The proposed mechanism of olefin silylation catalyzed by triangular metal clusters involves the olefin C−H bond activation on the triangular cluster core and formation of an intermediate which has a structure similar to that of cluster 1.