  Metal-fullerene compounds are characterized by significant electron transfer
to the fullerene cage, giving rise to an electric dipole moment. We use the
method of electrostatic beam deflection to verify whether such reactions take
place within superfluid helium nanodroplets between an embedded C$_{60}$
molecule and either alkali (heliophobic) or rare-earth (heliophilic) atoms. The
two cases lead to distinctly different outcomes: C$_{60}$Na$_n$ ($n$=1-4)
display no discernible dipole moment, while C$_{60}$Yb is strongly polar. This
suggests that the fullerene and small alkali clusters fail to form a
charge-transfer bond in the helium matrix despite their strong van der Waals
attraction. The C$_{60}$Yb dipole moment, on the other hand, is in agreement
with the value expected for an ionic complex.
