kontaktioonpaarid

Kontaktioonpaarid, known in English as contact ion pairs (CIPs), are pairs of oppositely charged ions that remain in direct contact in a solution, with no solvent molecule interposed between them. They represent one state in a spectrum that also includes solvent-separated ion pairs (SSIPs) and fully dissociated free ions. The relative stability of a CIP depends on factors such as ion charge density, solvent dielectric constant, concentration, temperature, and the solvent’s coordinating ability. CIP are favored when the electrostatic attraction between the ions is strong compared with the stabilizing effect of solvation; they are more common in low-dielectric or highly concentrated media and in solvents that poorly solvated ions. In highly polar solvents like water, CIP are less stable and SSIPs or free ions tend to predominate, though CIP can persist at high salt concentrations.

Detection and implications: CIP influence physical properties such as conductivity and reactivity. They are inferred from deviations in molar conductivity from the behavior expected for fully dissociated ions, as well as spectroscopic signatures and scattering data indicating ion pairing. In organic synthesis and catalysis, the existence of CIP can affect nucleophilicity, rate constants, and selectivity by maintaining intimate ion–ion contact. Disruption of CIP—by changing solvent, coordinating ligands, or increasing solvation—can shift the system toward SSIPs or free ions, altering reactivity.

Examples and context: In nonpolar solvents or in highly concentrated salt solutions, salts often exist predominantly as contact ion pairs or higher aggregates. In contrast, in aqueous or other highly polar media, CIP are less common except at high concentrations. In Estonian chemical literature, they are referred to as kontaktioonpaarid.