The structure of [Os(eta(2)-H-2)en(2)CH(3)CO(2)]PF6 (1) has been studied by X-ray diffraction at 120 K and neutron diffraction at 165 K. Compound 1 belongs to a series of dihydrogen complexes of osmium(II) amines, exhibiting an extensive range in the strength of the H-H interaction as judged by the wide variation in observed JH-D coupling constants. The ethylenediamine ligands in 1 have a trans arrangement. The H-2 and the acetate ligands occupy positions on opposite sides of the OsN4 plane, giving overall approximately octahedral coordination. Consistent with the relatively low J(H-D) value (9.0 Hz), the H-H distance in 1, 1.34(2) Angstrom as determined by neutron diffraction, is unusually long and approaches that of 1:357(7) Angstrom found in the polyhydride ReH7{P(p-tolyl)(3)}(2). It is presumed that 1 may represent an intermediate stage in the oxidative addition of dihydrogen at transition-metal centers. The Os-H distances in 1, 1.59(1) Angstrom and 1.60(2) Angstrom, are shorter than those in the classical osmium polyhydride OsH4(PMe(2)Ph)(3) (mean Os-H 1.659(3) W) lending support to the assertion that the Os(eta(2)-H-2) interaction is relatively strong. Crystal data : C6H19F6N4O2OSP, fw 514.4, monoclinic, P2(1)/n, Z = 4, a = 11.671(2) Angstrom, b = 10.639(2) Angstrom, c = 11.775(2) Angstrom, beta = 96.02(1)degrees at 165 K, d(calc) = 2.706 g cm(-1). Structure analysis : (X-ray) R(F) = 0.041 for 1603 reflections (F > 4 sigma(F)); (neutron) R(F) = 0.088 for 1897 reflections (F-2 > sigma(F-2)).