X-ray study of an osmium oxyfluoride of unknown composition

A small, colorless, single-crystal with appreciable vapor pressure at room temperature was presumed to be a new oxyfluoride compound of osmium. An X-ray study on a precession camera at a variety of temperatures from 20 to -100°C with many recrystallizations of the material revealed that it was dimorphic. One structure is monoclinic in space group C2/c with a unit cell nearly identical to that reported for OsO₄. The second structure is cubic, a = 8.595 Å, Z = 8, space group P3n, with an extraordinary assortment of systematic absences which yield the most remarkable diffraction patterns the writer has ever encountered. The observations are accounted for by a structure comprising 8 tetrahedral molecules with the anions in cubic close packing at an anion-anion distance of 3.04 Å and with an Os-anion distance of 1.86 Å. The most likely possibilities for the chemical composition appear to be OsO₂F₂ or OsO₃F.