The collision energy dependence of the cross section for the Penning ionization of C-60 molecules by metastable neon atoms has been measured in the 0.035-0.5 eV range in a crossed beam experiment, The results show a decreasing trend (similar to 20%) in the low-energy range up to 0.06 eV, with a slightly increasing dependence in the higher energy range. This behavior is interpreted in terms of the characteristics of the intermolecular interaction: an electron transfer can be possible from the metastable rare gas atom to the C60 molecule with the formation of an ion-pair collision complex. At low collision energy the system evolves mainly adiabatically and the ionization occurs mainly from the ion-pair complex. However, st higher collision energies, nonadiabatic transitions become possible and the systems can remain partially "trapped" within the higher adiabatic state with a consequent increase of the ionization probability.