Disorder determined by high-resolution powder diffraction: structure of pentamethylcyclopentadienyllithium.

The crystal structure of pentamethylcyclopentadienyllithium, [Li(C(10)H(15))] (LiCp*), has been determined from a high-resolution powder pattern by modelling and the maximum entropy method (MEM). The compound crystallizes in space group R3m with lattice parameters a = b = 14.7711 (5), c = 3.82206 (6) Å and V = 722.19 (4) Å(3) (Z = 3). LiCp* forms polymeric 'multidecker' chains along the c axis. The pentamethylcyclopentadienyl anions are coplanar with each other and show threefold rotational disorder. The MEM calculations did not only confirm the structural model and the type of disorder, but also discovered additional symmetry compared with the Rietveld analysis. This is the first solid-state structure of a Lewis-base-free alkali metal Cp* compound.