SYNTHESIS, STRUCTURE AND REACTIVITY OF DINUCLEAR HYDRIDO-BRIDGED OSMIUM COMPLEXES WITH OS-OS DOUBLE AND TRIPLE BOND

The dinuclear mesitylene osmium complexes [(Mes)2Os2(μ-H)3]X (2a,b) and [(Mes)OsH(μ-H)]2 (3) are prepared from [(Mes)OsCl2]n (1) and NaBH4. The complex 2a (X = Cl) and its corresponding BF4 salt 2c can also be obtained from the reaction of 3 with HCl or HBF4, respectively. The structures of 2a and 3 have been determined by single-crystal X-ray diffraction studies. The cation of 2a contains two equivalent (Mes)Os fragments linked by a rather short OsOs bond (247.41(2) pm) and three bridging hydride ligands. The midpoints of the six-membered rings together with the two osmium atoms form a linear unit. The metal-metal distance in compound 3 (267.34(6) pm) is significantly longer than that in the cation of 2a and corresponds to an OsOs double bond. The trans arrangement of the mesitylene rings with regard to the metal-metal bond axis indicates that two of the hydride ligands in 3 are terminal and two are bridging. An equilibrium between the cis and trans isomers of 3 probably exists in CD2Cl2 solution. Treatment of [(Mes)OsCl2(PMe3)] (4) with NaBH4 or LiAlH4 affords the dihydrido complex [(Mes)OsH2(PMe3)] (5) which is also one of the products of the reaction of 3 with PMe3. Compound 5 is photoreactive and on UV irradiation in benzene gives the hydrido phenyl complex [(Mes)OsH(C6H5)PMe3]. © 1990.