Difference in reactivity of triel halides EX3 towards ferrocene

A facile route to the Fe2+-arene complex [(C6H6)FeCp][AlBr4] is the reaction of ferrocene with AlBr3 in benzene. The structure of the Fe2+-arene complex [(C6H6)FeCp][AlBr4] · C6H6 was found to be isomorphous with those of [FeCp2][ECl4] · C6H6 (E = Al, Ga). The crystal structures of the [FeCp2][AlCl4] · C6H6 (E = Al, Ga) presented here show structural features which are different from those of solvent-free ferrocenium salts [FeCp2][ECl4] (E = Al, Ga, Fe). However, the cell parameters of solvent-free ferrocenium tetrafluoroborate [FeCp2][BF4] are also quite different from those of solvent-free [FeCp2][ECl4] (E = Al, Ga, Fe). In contrast to the eclipsed conformation in solvent-free [FeCp2][ECl4] (E = Al, Ga, Fe) the cyclopentadiene rings in [FeCp2][BF4] and [FeCp2][ECl4] · C6H6 (E = Al, Ga) are in a staggered conformation.