  The effects of interactions between He- and clusters of fullerenes in helium
nanodroplets are described. Electron transfer from $He^{-}$ to $(C_{60})_n$ and
$(C_{70})_n$ clusters results in the formation of the corresponding fullerene
cluster dianions. This unusual double electron transfer appears to be concerted
and is most likely guided by electron correlation between the two very weakly
bound outer electrons in $He^{-}$. We suggest a mechanism which involves long
range electron transfer followed by the conversion of $He^{+}$ into
$He_{2}^{+}$, where formation of the $He-He$ bond in $He_{2}^{+}$ releases
sufficient kinetic energy for the cation and the dianion to escape their
Coulombic attraction. By analogy with the corresponding dications, the
observation of a threshold size of n \geq 5 for formation of both
$(C_{60})_n^{2-}$ and $(C_{70})_n^{2-}$ is attributed to Coulomb explosion
rather than an energetic constraint. We also find that smaller dianions can be
observed if water is added as a co-dopant. Other aspects of $He^{-}$ chemistry
that are explored include its role in the formation of multiply charged
fullerene cluster cations and the sensitivity of cluster dianion formation on
the incident electron energy.
