Anion-Anion Interactions in Aerogen-Bonded Complexes. Influence of Solvent Environment.

Ab initio calculations are applied to the question as to whether a AeX5- anion (Ae = Kr, Xe) can engage in a stable complex with another anion: F-, Cl-, or CN-. The latter approaches the central Ae atom from above the molecular plane, along its C5 axis. While the electrostatic repulsion between the two anions prevents their association in the gas phase, immersion of the system in a polar medium allows dimerization to proceed. The aerogen bond is a weak one, with binding energies less than 2 kcal/mol, even in highly polar aqueous solvent. The complexes are metastable in the less polar solvents THF and DMF, with dissociation opposed by a small energy barrier.