A high-yield route to the new complexes OsBr2(dppe)(2) and trans-OsHBr(dppe)(2) starting from (NH4)(2)-[OsBr6] is described. The new 5-coordinate complexes [OsX(dppe)(2)]PF6 (X = Cl (8Os) and X = Br (9Os)) are prepared by reaction of cis-OsX(2)(dppe)(2) with NaPF6. Complexes 8Os and 9Os consist of distorted trigonal bipyramidal cations with "Y"-shaped equatorial planes. They react in CH2Cl2 with H-2 or HD 1 (atm) to give complexes trans-[Os(H ...)X(dppe)(2)]PF6 (X = Cl (1OsPF(6)), X = Br (2OsPF(6))) or trans-[Os(H ... D)X(dppe)(2)]PF6, respectively, The last complexes have J(H,D) = 13.9 and 13.7 Hz, respectively. The BF4- salts of these complexes, 1OsBF(4) and 2OsBF(4), respectively, are prepared by reacting trans-OsHX(dppe)(2) with HBF4 . Et(2)O or DBF4 . Et(2)O. These complexes are characterized by NMR, IR, and FAB MS. The single-crystal X-ray and neutron diffraction studies of 1OsPF(6) revealed an elongated H ... H ligand with d(HH) = 1.11(6) (X-ray) or 1.22(3) Angstrom (neutron) occupying one site in an octahedral complex. The X-ray diffraction study of 2OsBF(4) produced a similar structure with d(HH) = 1.13(8) Angstrom. One fluorine of the anion in each structure is positioned near the acidic H-2 ligand. A linear relationship between d(HH) and J(H,D) fur many dihydrogen complexes is used to indicate that complexes 1Os and 2Os have H-H distances of about 1.2 Angstrom in solution. Plots of ln(T-1) versus inverse temperature for 1Os and 2Os are distorted from the usual "V" shape, suggesting that the rotational frequency of the H-2 ligand is near that of the Larmor frequency. Therefore the d(HH) for 1Os is between the values of 1.04 and 1.31 Angstrom calculated from the T-1(min) for fast and slow spinning H-2, respectively. The chloride ligand in trans-[Os(H ... H)Cl(L)(2)](+) buffers the effect of changing the cis ligands L from dppe to depe to dcpe so that there is little change in the H-H distance. Complexes 1Os and 2Os have pK(a) values of 7.4 and 5.4 respectively, while trans-[Os(H-2)H(dppe)(2)](+) is much less acidic with a pK(a) of 13.6. These pK(a) values and some E(1/2) values are used to show that 1Os and 2Os are dihydride-like even though they have relatively short H-H distances. Properties of trans-[Os(H ... H)Cl(depe)(2)]BF4 (3Os) are also reported.