Multi-center covalency: revisiting the nature of anion-π interactionsElectronic supplementary information (ESI) available: Fig. S1: bonding molecular orbitals of selected complexes. Fig. S2: structures of complexes with the atom-numbering scheme. Table S1: delocalization indices, electrostatic and exchange-correlation contributions to binding energy. See DOI: 10.1039/c5cp05777a

Exploring the nature of anion-π 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-π systems to closely related covalent anion-σ complexes reveals that the anion-π systems benefit from an extensive degree of electron sharing between the anions and all atoms of the π-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-π 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-π complexes with the anions atop the π-rings might be prepared with π-systems that benefit more from the exchange-correlation term, such as extended π-systems, but not with strong electrostatic π-receptors. This conclusion is in line with the tendency of strong π-acids to form the σ-complexes with more covalent character instead of the π-complexes. Plot of the delocalization index, δ (X − ,Ω), scaled by the internuclear distance, R X,Ω , versus the exchange-correlation potential energy, V XC (X − ,Ω), for anion-π complexes.