The classical Os-VI hexahydride ((PPr3)-Pr-i)(2)OsH6 (1) undergoes a chemically irreversible oxidation at a remarkably low oxidation potential E(p) = 0.77 V vs Cp(2)Fe/Cp(2)Fe(+) (cyclic voltammetry, Au electrode, acetonitrile/ 0.1 M Bu(4)N(+)PF(6)(-)). Chemical oxidation with 1 equiv of acetylferrocenium tetrafluoroborate in dichloromethane generates ((PPr3)-Pr-i)(2)OsH3(H-2)(2)(+) (4) as a major product, presumably by proton transfer from the Bronsted acid 1(.+) to 1. Compound 4 is also available by treatment of 1 with HBF4; 1 is regenerated by the addition of piperidine. In acetonitrile, 4 undergoes loss of H-2 to give ((PPr3)-Pr-i)(2)Os(NCMe)(2)H-3(+) (2), believed to probably assume a classical trihydride structure. Further reaction with acetonitrile leads to ((PPr3)-Pr-i)(2)Os(NCMeH+ (3); quite remarkably, this reaction can be reversed when one acetonitrile ligand is displaced by H-2. The cationic hydrides 2 and 3 do not undergo proton transfer to amine bases; rather, both can be protonated by HBF4 to give the dicationic complexes ((PPr3)-Pr-i)(2)Os(NCMe)(2)H-4(2+) (5, with one or two eta(2)-H-2 ligands) and ((PPr3)-Pr-i)(2)Os(NCMe)3(Hz)2+ (6), respectively. These reactions are reversed when piperidine is added. The polyhydride complexes have been characterized by H-1 NMR spectroscopy by T-1min measurements and by measurements of J(HD) values for partially deuterated samples.