Donor-Acceptor Interaction and the Peculiar Structures of Dications

The geometries and stabilities of dications are explained by the donor-acceptor interaction of a (neutral) donor and an (doubly charged) acceptor molecule, respectively. The bonding in these donor-acceptor complexes is analyzed by means of one-electron density analysis. A simple model is presented to rationalize the bonding features of dications CH₂X²⁺ and CH₄X²⁺. Depending on the type and number of donor molecules, three cases of donor-acceptor complexes can be distinguished. Type I complexes comprise dications where the electron acceptor CH₂²⁺ is bound to molecule × which donates electronic charge via a lone-pair orbital. The second class (type II complexes) consists of species where electron donation of × arises from a bonding σ-MO, and type III complexes occur when two donor molecules, such as H₂ and × in CH₄X²⁺, donate electronic charge into CH₂²⁺. It is found that the geometries and stabilities of the three classes of dications can be explained by the strength and type of orbital interaction between donor and acceptor, respectively. The stabilization due to electron donation from two donors × and H₂ in type III dications is not simply additive but rather depends on the actual orbitals being involved in the interaction. The model presented here can be used to predict stable structures for unknown dications.