Beyond Classical Ion Pairing: Hydrogen Bonding, Reverse Coordination, and a New Framework for Weakly Solvating Electrolytes

Ion association in solution has traditionally been described in terms of electrostatic interactions; however, its molecular origin in weakly solvating media remains insufficiently understood. In this Perspective, we systematically revisit ion-association reactions in aprotic solvents based on hydrogen bonding and electron pair donor–acceptor interactions, drawing upon conductometry, polarography, spectroscopic measurements, and DFT calculations. We demonstrate that in weakly solvating media, such as acetonitrile and benzonitrile, a variety of specific association species are formed beyond simple ion pairs. Homo- and heteroconjugated species of the types A−(HA)2 and A−(HX)2 are widely observed with large formation constants, indicating strong, cooperative hydrogen bonding. In parallel, “reverse coordination” complexes such as A−(Li+)2 are identified, revealing that a single anion can function as a multidentate donor toward multiple metal cations. Furthermore, mixed association species involving both hydrogen bonding and coordination interactions, exemplified by HA2−Li+, are elucidated, highlighting the competitive and cooperative interplay between proton donors and metal ions. Conductometric analyses provide clear evidence for the formation of higher-order ionic aggregates, including symmetric triple ions and quadrupolar species in solvents with relatively high permittivities (εr ≈ 20–65). In addition, studies on disulfonic acids reveal that multiple hydrogen bonds act cooperatively to generate highly stabilized “double-handed” homoconjugated structures, supported by spectroscopic data and DFT calculations. This conceptual framework provides new design principles for next-generation highly concentrated and weakly solvating battery electrolytes.