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 C₂F₄²⁺, 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(tt) 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.