SUBSTITUTED ETHYLENE DICATIONS - PLANAR OR PERPENDICULAR

Neutral and doubly charged substituted ethylene dications C2X2Y2 are theoretically studied by use of ab initio quantum theoretical methods. Planar geometries are predicted for the dications C2F4(2+), C2(OH)4(2+), C2H2(NH2)2(2+) and C2(OH)2(NH2)2(2+), while twisted structures are calculated for C2H4(2+), C2(NH2)4(2+), and C2(SH)4(2+). The results are explained with molecular orbital arguments. Substituents X with lone-pair orbitals strongly donate electronic charge into the formally empty C(pi) orbital of C2X4(2+), yielding partial CX double bonds. If X is a first-row atom, conjugation of the resulting double bonds causes a planar geometry as energy minimum form if steric repulsion of the vicinal groups is absent. The experimentally observed twisted geometry of the bromo and chloro salts of C2(NH2)4(2+) cannot be considered as validation that C2H4(2+) has a perpendicular energy minimum geometry.