The C-F . . .M+ interaction was investigated by observation of the NMR spectroscopic changes and complexation studies between metal cations and the cage compounds 1 and 2 which have fluorobenzene units as donor atoms. As a result, the interaction was detected with. all of the metal cations which form. complexes with 1 and 2. The stability of the complexes of I and 2 was determined by the properties of the metal cations (ionic radii and degree of hydrolysis), not by the hard-soft nature of the cations. Crystallographic analyses of Tl+ subset of 1 and La3+ subset of 2 provided structural information (interatomic distances and bond angles), and the bond strengths, C-F . . .M+, O . . .M+, and N . . .M+, were estimated by Brown's equation based on the structural data. Short C-F . . . Tl+ (2.952-3.048 Angstrom) distances were observed in the complex Tl+ subset of 1. The C-F bond lengths in the complexes, Tl+ subset of 1 and La3+ subset of 2, are elongated compared to those of the metal-free compounds. Interestingly, no solvent molecules including water molecules were coordinated to La3+ in the La3+ subset of 2. The stabilization energy of cation-dipole interaction was calculated on the basis of the data from X-ray crystallographic analysis, and it is roughly consistent with the -DeltaH values estimated in solution. Thus, the C-F . . .M+ interaction can be expressed by the cation-dipole interaction. This result explains the fact that compound I which has fluorine atom as hard donor strongly binds soft metals such as Ag+ and Tl+. Furthermore, it was concluded that the fluorobenzene unit has a poor electron-donating ability compared to that of ether oxygen or amine nitrogen, and thus the ratio of the coordination bond in C-F . . .M+ is small. The specific and remarkable changes in the H-1, C-13, and F-19 NMR spectra were observed accompanied by the complexation between M+ and the hosts 1 and 2. These spectral features are important tools for the investigation of the C-F . . .M+ interaction. Furthermore, F . . . Tl+ spin couplings were observed at room temperature in the Tl+ subset of 1, 2 (J(F-Tl) = 2914 Hz for Tl+ subset of 1 and 4558 Hz for Tl+ subset of 2), and these are clear and definitive evidence of the interaction.