  Charge-state transitions of a single Cu-phthalocyanine molecule adsorbed on
an insulating layer of NaCl on Cu(111) are probed by means of alternate
charging scanning tunneling microscopy. Real-space imaging of the electronic
transitions reveals the Jahn--Teller distortion occurring upon formation of the
first and second anionic charge states. The experimental findings are
rationalized by a theoretical many-body model which highlights the crucial role
played by the substrate. The latter enhances and stabilizes the intrinsic
Jahn--Teller distortion of the negatively charged molecule hosting a degenerate
pair of single-particle frontier orbitals. Consequently, two excess electrons
are found to occupy, in the ground state, the same localized orbital, despite a
larger Coulomb repulsion than the one for the competing delocalized electronic
configuration. Control over the charging sequence by varying the applied bias
voltage is also predicted.
