Oxo complexes of osmium(IV) formed via dioxygen activation. X-ray structures of [OsX(dcpe)(2)]PF6 (X = Cl, Br), [OsCl(eta(2)-O-2)(dcpe)(2)]BPh4, and [OsCl(O)(dcpe)(2)]BPh4 (dcpe=1,2-bis(dicyclohexylphosphino)ethane)

Dioxygen addition to the 16-electron complexes [OsX(P-P)(2)](+) (3) gives t he dioxygen adducts [OsCl(eta (2)-O-2)(P-P)(2)](+) (3), which in turn react with HCl gas to give the novel osmium(IV) oxo complexes trans-[OsX(O)(P-P) (2)](+) (5) (X = Cl, Br; P-P = 1,2-bis(dicyclohexylphosphino)ethane (dcpe), 1,2;bis(diethylphosphino)ethane (depe), 1,2-bis((2R,5R)-2,5-dimethylphosph opholano) (Me-duphos)). The complexes [OsX(dcpe)(2)](+) (X = Cl, Br) (3) ar e studied by X-ray crystallography and are shown to have a "Y-shaped" coord ination geometry in the equatorial plane. The X-ray structural analysis of [OsCl(eta (2)-P-2)(dcpe)(2)](+) (4a) reveals an exceptionally short O-O bon d (1.315(5) Angstrom). trans-[OsCl(O)(dcpe)(2)](+) (5a), the first oxo comp lex of osmium(IV) investigated crystallographically, exhibits a long Os-O d istance of 1.834(3) Angstrom. The reactivity of 4 and 5 as oxidants is desc ribed. The dioxygen complex 4a transfers one oxygen atom to PPh3 (to give P h3PO) or oxidizes iodide ions to triiodide ions in the presence of anhydrou s HCl. In both reactions, the corresponding oxo species 5a is quantitativel y formed as the only metal-containing product. Oxo complexes 5 are surprisi ngly stable and unreactive toward standard reducing agents such as phosphin es.