DIHYDRIDO AND TRIHYDRIDO DIOLEFIN COMPLEXES STABILIZED BY THE OS(PIPR(3))(2) UNIT - NEW EXAMPLES OF QUANTUM-MECHANICAL EXCHANGE COUPLING INTRIHYDRIDO OSMIUM COMPOUNDS

The hexahydrido complex OsH6(PiPr(3))2 (1) reacts with tetrafluorobenz obarrelene (TFB), 2,5-norbornadiene (NBD), and 1,3-cyclohexadiene to a fford OsH2(eta(4)-TFB)(PiPr(3))(2) (2), OsH2(eta(4)-NBD)(PiPr(3))(2) ( 3), and OsH2(eta(4)-cyclohexadiene)(PiPr(3))(2) (4), respectively. The protonation of 2 and 3 with HBF4 yields [OsH3(eta(4)-TFB)(PiPr(3))(2) ]BF4 (5) and [OsH3(eta(4)-NBD)(PiPr(3))(2)]BF4 (6) The H-1 NMR spectra of 5 and 6 in the hydride region at low temperature display AM(2)X(2) spin systems (X = P-31), which are simplified to AM(2) spin systems i n the H-1{P-31} spectra. The values for J(AM) are temperature dependen t, increasing from 13.1 to 35.9 Hz (5) and from 11.0 to 17.7 Hz (6) as temperature is increased from 190 to 230 K and from 180 to 240 K, res pectively. The reaction of 4 with HBF4 leads to the cyclohexenyl compl ex [OsH2(eta(3)-C6H9)(PiPr(3))(2)]BF4 (7), which shows an agostic inte raction between the osmium center and one of the two endo-CH bonds adj acent to the pi-allyl unit. In solution complex 7 is fluxional. The fl uxional process involves the exchange between the relative positions o f the hydride ligands and the endo-CH hydrogen atoms of the cyclohexen yl ligand and, at the same time, the exchange between the CH allyl and the exo-CH hydrogen atoms inside the cyclohexenyl ligand. The structu re of 7 in the solid state has been determined by X-ray diffraction. T he distribution of ligands around the osmium atom can be described as a piano stool geometry with the agostic hydrogen atom and the midpoint s of the carbon-carbon bonds involved in the pi-allyl unit forming the three-membered face, while both the hydride and phosphine ligands lie in the four-membered face.