  Quantum tunneling dominates the low temperature magnetization dynamics in
molecular magnets and presents features that are strongly system dependent. The
current discussion is focused on the terbium(III) bis(phtalocyanine)
([TbPc$_2$]$^{-1}$) complex, that should serve as a prototypical case for
lanthanide molecular magnets. We analyze numerically the effect of non-axial
interactions on the magnitude of the intrinsic tunnel splitting and show that
usual suspects like the transverse ligand field and Zeeman interaction fail to
explain the experimentally observed dynamics. We then propose through the
nuclear quadrupolar interaction a viable mechanism that mixes, otherwise
\textit{almost} degenerate hyperfine states.
