REACTIONS OF ((PPR3)-PR-I)(2)OSH6 INVOLVING ADDITION OF PROTONS AND REMOVAL OF ELECTRONS - CHARACTERIZATION OF ((PPR3)-PR-I)(2)OS(NCME)(X)H-Y(Z+) (X=0, 2, 3 Y=1, 2, 3, 4, 7 Z=1, 2), INCLUDING DICATIONIC ETA(2)-H-2 COMPLEXES

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. Thus, the H-H distance in 4 is estimated as 1.00 Angstrom (0.79 Angstrom, fast-spinning). For 2, the T-1min and J(HD) leaves it in the uncertain range between classical and nonclassical hydrides. Assuming a hydride/dihydrogen structure, a H-H distance of 1.40 Angstrom (1.11 Angstrom, fast-spinning) is calculated, indicating a dihydrogen ligand at or beyond the brink of cleavage. However, a trihydride classical structure is favored due to the relatively slow reaction of 2 with acetonitrile. The H-H distance in 5 is 1.09 Angstrom (0.86 Angstrom, fast-spinning) assuming a bis-(eta(2)-H-2) structure, or 0.97 Angstrom (0.77 Angstrom, fast-spinning) for a (eta(2)-H-2)(H)(2) structure. For 6, the H-H distance is 1.09 Angstrom (0.87 Angstrom, fast-spinning).