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The unique linear triosmium clusters OS~B~(CO)~O(CNR)(~~~-C~H~) (2) are prepared bywith allyl bromide. The reaction rate is faster in CH2-Cl2 than in cyclohexane. The oxidative addition of allyl bromide to the triosmium cluster is likely to involve a partial heterolytic metal-metal bond fission pathway, and the coordinated isocyanide ligand plays a n important role in determining the regioselectivity and the transformation of allyl bromide in the coordination sphere of the cluster. I n the absence of the coordinated isocyanide ligand, complex OsdCO)11(NCMe) reacts with allyl bromide in CH2Cl2 to afford a similar linear complex Os3Br(CO)ll(y3-C3H5) (4) with a slower rate. Complex OS~B~(CO)~~(CNP~)(~~-C~H~) (2a) crystallized in the triclinic space group Pi with a = 10.518(5) A, b = 12.026(2) A, c = 19.116(3) A, a = 89.98(1)", B = 90.13(3)", y = 93.23-(3)", V = 2414(1) A3, 2 = 4, R = 4.7%, and R, = 5.1% for 3810 observed reflections. Boththe bromide and the isocyanide ligands coordinate equatorially at the first Os atom, while the allyl group occupies an axial and an equatorial site on the third Os metal center in the linear triosmium cluster.

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