The six-coordinate diamagnetic osmium(1V) complexes O S H~C I~( P R~)~ [PR, = P-i-Pr, (Z), PMe-?-Bu2 (3)] are prepared from OsC1,.xH20 and PR, in boiling 2-propanol in ca. 80% yield. Treatment of 3 with CO leads to reductive elimination of H2 and formation of aIl,rran.~-OsC1~(CO)~(PMe-f-B~~)~ (4). Likewise, 3 reacts with excess PMe, to give ci~-OsCl~(PMe~)~ (5). The crystal and molecular structure of 2 has been determined. Crystal data for 2: monoclinic, P2,/c, 4 = 12.2791 (6) A, b = 8.5700 (3) A, c = 23.5061 (9) A, fi = 103.526 (4)O, Z = 4, R = 0.022, R, = 0.024 based on 3705 observed unique reflections. The coordination polyhedron around the six-coordinate osmium atom is described as a somewhat distorted variant of the Du square antiprism with two vacant coordination sites in alternate positions at one square base of this polyhedron. The reaction of 4 withLiAIH, in THF leads to the formation of all,tr~ns-OsH~(CO)~(PMe-i-B~~)~ (6), whereas treatment of 2 and 3 with NaBH, in methanol/benzene affords the hexahydrido complexes OsH6(PR& in ca. 60% yield. These hexahydrides are probably catalyst precursors for hydrogen transfer from 2-propanol to ketones such as cyclohexanone, methylcyclohexanones, and acetophenone catalyzed by 2 and 3 in presence of NaBH,. Compound 2 catalyzes the hydrogenation of olefins and dienes such as 1,5-and 1 ,3-C8HI2 at considerable initial rates. Benzylideneacetone and benzylideneacetophenone are also reduced with 2 as catalyst to the saturated ketones with high selectivity.