Collective Interactions in Ion Pairs

Collective interactions represent a recently proposed bonding mode in which stabilization arises not from a localized two-center bond but from the distributed exchange-correlation effects between a central site and several surrounding atoms, whereas the interaction between the central atom and the metal is destabilizing. While this phenomenon has been demonstrated in certain organometallic complexes, its possible existence in predominantly ionic systems remains unexplored. Here, we present a comprehensive study on a family of ion pairs of general formula M[AX₄] (M = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺; A = B, Fe Co, Zn, Cd; X = -CH₃, -CO, -CCH, -OCH₃, -Cl) aimed at determining whether collective interactions can arise in systems conventionally described as ionic. Through a combination of penetration index analysis and Interacting Quantum Atoms (IQA) decomposition, we quantify the contributions of Coulombic and exchange-correlation terms to the total interaction energy. The calculated exchange-correlation interaction collectivity indices (ICI_XC) reveal that alkali metal-tetramethylborate ion pairs and several transition metal tetrahedral anions exhibit distinctly collective behavior, particularly when complexes possess electron-rich or polarizable ligands (-CCH, -OCH₃, -Cl). In contrast, carbonyl-containing systems display larger ICI_XC values, consistent with a classical noncollective bonding scheme. These results demonstrate that collective interactions can indeed manifest in ionic environments, extending the scope of the concept beyond covalent or organometallic frameworks and offering new insights into the fundamental nature of ion pairing and electronic delocalization.