PROMOTION EFFECT OF COORDINATED ISOCYANIDE ON THE OXIDATIVE ADDITION OF ALLYL BROMIDE IN TRIOSMIUM CLUSTERS

The unique linear triosmium clusters Os3Br(CO)10(CNR)(eta3-C3H5) (2) a re prepared by reaction of Os3(CO)10(CNR)(NCMe) with allyl bromide. Th e reaction rate is faster in CH2-CL2 than in cyclohexane. The oxidativ e addition of allyl bromide to the triosmium cluster is likely to invo lve a partial heterolytic metal-metal bond fission pathway, and the co ordinated isocyanide ligand plays an important role in determining the regioselectivity and the transformation of allyl bromide in the coord ination sphere of the cluster. In the absence of the coordinated isocy anide ligand, complex Os3(CO)11(NCMe) reacts with allyl bromide in CH2 Cl2 to afford a similar linear complex Os3Br(CO)11(eta3-C3H5) (4) with a slower rate. Complex Os3Br(CO)10(CNPr)(eta3-C3H5) (2a) crystallized in the triclinic space group P1BAR with a = 10.518(5) angstrom, b = 1 2.026(2) angstrom, c = 19.116(3) angstrom, a = 89.98(1)-degrees, beta = 90.13(3)-degrees, gamma = 93.23(3)-degrees, V = 2414(1) angstrom3, Z = 4, R = 4.7%, and R(w) = 5.1% for 3810 observed reflections. Both th e bromide and the, isocyanide ligands coordinate equatorially at the f irst Os atom, while the allyl group occupIes an axial and an equatoria l site on the third Os metal center in the linear triosmium cluster.