Ab Initio Structure Determination of Two Polymorphs of Cyclopentadienylrubidium in a Single Powder Pattern

The structures of two polymorphic phases of solid RbC₅H₅ have been solved ab initio by high-resolution powder X-ray diffraction measured in a single powder pattern. The compound crystallizes in space group Pbcm (phase I), Z = 8, with unit-cell parameters a = 9.3396 (1), b = 10.9666 (1), c = 10.5490 (1) Å, and in space group Pnma, Z = 4 (phase II), with unit- cell parameters a = 10.7990 (2), b = 8.6923 (2), c = 5.7061 (2) Å. Both phases show polymeric zigzag chains, so-called bent-polydecker sandwich structures, with an. average Rb-Cp_z (Cp-ring-centroid) distance of 2.97 Å. Phase II contains a single chain along the a axis, whereas phase I contains two similar but crystallographically independent chains perpendicular to each other along the b axis and along the c axis, leading to a more complex coordination sphere around the Rb atoms. The chains are bent with Cp_z-Rb-Cp_z angles ranging from 123.5 to 136.5°.