Fig. 2. Frontier molecular orbitals of computational models 1′ and 1′′. (a) HOMO−3 δ-bond (164, −5.093 eV) of 1′, (b) HOMO−2 δ-bond (165, −5.036 eV) of 1′, (c) HOMO−1 π-bond (166, −4.218 eV) of 1′, (d) HOMO π-bond (167, −4.119 eV) of 1′, (e) HOMO−1 π-bond (195, 3.018 eV) of 1′′, (f) HOMO σ-bond (196, 3.145 eV) of 1′′. The positive energies of the HOMO and HOMO−1 of 1′′ reflect the formal 2− charge applied to the system, but the electrons are clearly not detached. Hydrogen atoms are omitted for clarity.