  So far, no boron fullerenes were synthesized: more compact sp3-bonded
clusters are energetically preferred. To circumvent this, metallic clusters
have been suggested by Pochet et al. [Phys. Rev. B 83, 081403(R) (2011)] as
"seeds" for a possible synthesis which would topologically protect the sp2
sector of the configuration space. In this paper, we identify a basic
pentagonal unit which allows a balance between the release of strain and the
self-doping rule. We formulate a guiding principle for the stability of boron
fullerenes, which takes the form of an isolated filled pentagon rule (IFPR).
The role of metallic clusters is then reexamined. It is shown that the
interplay of the IFPR and the seed-induced doping breaks polymorphism and its
related problems: it can effectively select between different isomers and
reduce the reactivity of the boron shells. The balance between self and
exterior doping represents the best strategy for boron buckyball synthesis.
