  A singular evolution toward an insulating phase, shown by 23Na and 13C NMR,
has been observed in the superconducting fullerides (NH3)xNaK2C60 for x>1.
Unlike most common cases, this insulating phase is non magnetic and 13C spin
lattice relaxation shows the presence of a spin gap. These two features suggest
that a charge disproportion from C60^(3-) to C60^(2-) and C60^(4-) can drive
the system from the metallic to the insulating state.
  The restoring of the Na+ cation in the center of the octahedral interstice in
the insulating phase, as indicated by 23Na and 2H lineshape analysis, confute
the current belief that the cation off-centering is effective in quenching the
superconductivity.
