|
In chemistry, ion-association is a chemical reaction whereby ions of opposite electrical charge come together in solution to form a distinct chemical entity. Ion-associates are classified, according to the number of ions that associate with each other, as ion-pairs, ion-triplets, etc. Ion-pairs are also classified according to the nature of the interaction as contact, solvent-shared or solvent-separated. The most important factor to determine the extent of ion-association is the dielectric constant of the solvent. Ion-associates have been characterized by means of vibrational spectroscopy. == Classification == ''Ion-pairs'' are formed when a cation and anion come together. : An+ + Bm− AB(n-m)+ There are three distinct types of ion-pair, depending on the extent of solvation of the two ions. Image:solvent separated.png| fully solvated Image:solvent shared.png| solvent-shared or solvent-separated Image:contact ion pair.png| contact In the schematic representation above, the circles represent spheres. The sizes are arbitrary and not necessarily similar as illustrated. The cation is coloured red and the anion is coloured blue. The green area represents solvent molecules in a primary solvation shell: secondary solvation is ignored. When both ions have a complete primary solvation sphere the ion-pair may be termed ''fully solvated''. When there is about one solvent molecule between cation and anion, the ion-pair may be termed ''solvent-shared''. Lastly when the ions are in contact with each other the ion-pair is termed a ''contact'' ion-pair. Even in a contact ion-pair, however, the ions retain most of their solvation shell. The nature of this solvation shell is generally not known with any certainty. In aqueous solution and in other donor solvents, metal cations are surrounded by between four and nine solvent molecules in the primary solvation shell,〔 Chapter 5, "Solvation numbers".〕 but the nature of solvation of anions is mostly unknown. An alternative name for a solvent-shared ion-pair is an ''outer-sphere complex''. This usage is common in co-ordination chemistry and denotes a complex between a solvated metal cation and an anion. Similarly a contact ion-pair may be termed an ''inner-sphere complex''. The essential difference between the three types is the closeness with which the ions approach each other; fully solvated > solvent-shared > contact. With fully solvated and solvent shared ion-pairs the interaction is primarily electrostatic, but in a contact ion-pair there will also be some covalent character in the bond between cation and anion. An ''Ion-triplet'' may be formed from one cation and two anions, or from one anion and two cations. Higher aggregates, such as a tetramer, (AB)4 may be formed. Ternary ion-associates involve the association of three species. Another type, named ''intrusion ion-pair'' has also been characterized. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ion-association」の詳細全文を読む スポンサード リンク
|