In chemistry the ion pair concept (introduced by Saul Winstein) describes the interactions between a cation, anion and surrounding solvent molecules. In an ordinary aqueous solutions of inorganic salts an ion is completely solvatated and shielded from the counterion. In less polar solvents two ions can stll be connected to some extent. In a tight or intimate or contact ion pair there are no solvents molecules between the two ions. When solvatation increases ionic bonding decreases and a loose or solvent-separated ion pair results. The ion pair concept explains stereochemistry in solvolysis.

references

Salt Effects and Ion Pairs in Solvolysis and Related Reactions. III.1 Common Ion Rate Depression and Exchange of Anions during Acetolysis S. Winstein, E. Clippinger, A. H. Fainberg, R. Heck, G. C. Robinson; J. Am. Chem. Soc.; 1956; 78(2); 328-335.