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5. Note added in proof: After submission of this paper the crystal structure analysis of the 2:1 ?-CD: ferrocene adduct was published (Y. Odagaki, K. Hirotsu, T. Higuchi, A. Harada, and S. Takahashi:J. Chem. Soc., Perkin Trans. I 1230 (1990)). This structure is isomorphous to the 2:1 ?-CD adduct of the ferrocenium salt we report here. It is noteworthy that the two Cp-rings of the guest molecule in the structure reported by Hirotsu et al. adopt a staggered arrangement, whereas we observe a nearly eclipsed conformation similar to that of free ferrocene. The calculated model [4] detects essentially no energy difference between eclipsed and staggered ferrocene.