One monolayer Of C-60 on one monolayer of hexagonal boron nitride on nickel is investigated by photoemission. Between 150 and 250 K the work function decreases and the binding energy of the highest-occupied molecular orbital increases by \&AP; 100 meV. In parallel, the occupancy of the-in the cold state almost empty-lowest unoccupied molecular orbital (LUMO) changes by 0.4\&PLUSMN; 0.1 electrons. This charge redistribution is triggered by the onset of molecular rocking motion, i.e., by orientation dependent tunneling between the LUMO Of C-60 and the substrate. The magnitude of the charge transfer is large and cannot be explained within a single-particle picture. It is proposed to involve electron-phonon coupling where C-60(-) polaron formation leads to electron self-trapping.