Multi-center covalency: revisiting the nature of anion-π interactions

Exploring the nature of anion-pi bonding by means of the Quantum Theory of Atoms in Molecules (QTAIM) and an energy decomposition scheme on the basis of Interacting Quantum Atoms (IQA) theory led us to conclude that these non-classical interactions benefit from multi-center covalency'' far more than from the electrostatics. Comparing anion-pi systems to closely related covalent anion-sigma complexes reveals that the anion-pi systems benefit from an extensive degree of electron sharing between the anions and all atoms of the pi-rings. Besides, decomposition of the binding energy into classical (electrostatics) and non-classical (exchange-correlation) components demonstrates that in contrast to previous reports, the anion-pi complexes are local minima, if and only if the non-classical contribution to binding energy surpasses that of the electrostatics. This suggests that the stable anion-pi complexes with the anions atop the pi-rings might be prepared with pi-systems that benefit more from the exchange-correlation term, such as extended p-systems, but not with strong electrostatic pi-receptors. This conclusion is in line with the tendency of strong pi-acids to form the sigma-complexes with more covalent character instead of the pi-complexes.