AB-INITIO STRUCTURE DETERMINATION OF 2 POLYMORPHS OF CYCLOPENTADIENYLRUBIDIUM IN A SINGLE POWDER PATTERN

The structures of two polymorphic phases of solid RbC5H5 have been sol ved 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 = 10.96 66 (1), c = 10.5490 (1) Angstrom, and in space group Pnma, Z = 4 (phas e II), with unit cell parameters a = 10.7990 (2), b = 8.6923 (2), c = 5.7061 (2) Angstrom. Both phases show polymeric zigzag chains, so-call ed bent-polydecker sandwich structures, with an average Rb-Cp(z) (Cp-r ing-centroid) distance of 2.97 Angstrom. Phase II contains a single ch ain along the a axis, whereas phase I contains two similar but crystal lographically independent chains perpendicular to each other along the b axis and along the c axis, leading to a more complex coordination s phere around the Rb atoms. The chains are bent with Cp(z)-Rb-Cp(z) ang les ranging from 123.5 to 136.5 degrees.