The HREELS spectra of a C-60 monolayer deposited on Ag(111) show evidences for a strong chemical bonding with the Ag substrate, accompanied by a charge transfer of about 1 electron per adsorbed molecule. K atom intercalation in the adsorbed layer allows an easy tuning of the C-60 LUMO (t(1u)) occupancy. The evolution of the C-60 intramolecular vibrational spectrum upon K intercalation is studied by HREELS and discussed with particular emphasis on the two A(g) (Raman active) and four T-1u (IR active) modes. Both A(g) modes exhibit at the interface dipolar activity explained by electron-phonon coupling. Upon K intercalation, the T-1u modes show an evolution similar to what is observed in K- or Rb-doped bulk C-60, allowing to identify C-60(6-) as the saturated phase.