Ferrocenium Tetrachloroferrate Revisited, and Ferrocenium Tetrachloroaluminate

The structure of the title compound, [Fe(Cp)₂][FeCl₄] (Cp = cyclopentadienyl, C₅H₅), was reported in 1978 in the non-centrosymmetric space group Pna2₁ at room temperature [Paulus & Schäfer (1978). J. Organomet. Chem. 144, 205-213]. A low-temperature data set indicates a phase transition from the non-centrosymmetric space group to the centrosymmetric Pnma. Crystals of [Fe(Cp)₂][AlCl₄] have also been studied at low temperature and are isomorphous with the FeCl₄^- complex.

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