Determination of the Structure of [Os(η²-H₂)en₂CH₃CO₂]PF6 by X-ray and Neutron Diffraction

The structure of [Os(η²-H₂)en₂CH₃CO₂]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 J_h-d coupling constants. The ethylenediamine ligands in 1 have a trans arrangement. The H₂ 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) Å as determined by neutron diffraction, is unusually long and approaches that of 1.357(7) Å 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) Å and 1.60 (2) Å, are shorter than those in the classical osmium polyhydride OsH₄(PMe₂Ph)₃ (mean Os-H 1.659(3) Å) lending support to the assertion that the Os(η²-H₂) interaction is relatively strong. Crystal data: C₆H₁₉F₆N₄O₂OsP, fw 514.4, monoclinic, P2₁/n, Z = 4, a = 11.671(2) Å, b = 10.639(2) Å, c = 11.775(2) Å, β = 96.02(1)° at 165 K, d_calc = 2.706 g cm^-1. Structure analysis: (X-ray) R(F) = 0.041 for 1603 reflections (F > 4σ(F)); (neutron) R(F) = 0.088 for 1897 reflections (F² > σ(F²)).