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 JH-D coupling constants. The ethylenediamine ligands in I 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 JH-D value (9.0 Hz), the H-H distance in 1.134(2) A as determined by neutron diffraction, is unusually long and approaches that of 1.357(7) A found in the polyhydride ReH7{P(p-tolyl)3}2. It is presumed that I may represent an intermediate stage in the oxidative addition of dihydrogen at transition-metal centers. The Os-H distances in 1.1.59(1) A and 1.60(2) A, are shorter than those in the classical osmium polyhydride OsH4(PMe2Ph)3 (mean Os-H 1.659(3) A) 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) A, b = 10.639(2) c = 11.775(2) A, β = 96.02(1)° at 165 K, dealc = 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)).