Ionic aggregates of 1-1 salts in nonaqueous solutions: Structure, thermodynamics and solvation

Abstract

The field of ionic aggregates finds its place at the boundary between more classical topics in chemistry such as ion, fused salt, coordination and organometallic chemistry. Ionic aggregates are found in very different situations : unsolvated in the gas state, solvated in solid solvates and in solution. In the case of model compounds like LiBr and LiSCN we now have a complete knowledge on the structure and the properties of aggregates in solution. Differences between unsolvated and solvated aggregates arise from the fact that in solvated aggregates the coordination number of lithium is usually equal to 4. As a consequence two main states of aggregation beyond ion pairs are predominent in solution : dimers and tetramers while trimers have not been identified. Ion pairs are strongly polar while dimers and tetramers are non polar. A cubane like structure is the rule in tetramers. Dimerization equilibria are non ideal because there are strong dipole dipole interactions between ion pairs. The three steps of aggregation are entropy driven reactions. Each reaction is a competition between solvent and anion in the first solvation shell of the cation. The solvation numbers of lithium have been measured by infrared in solvent-benzene mixtures. Dimerization equilibria of NaNCS and KNCS are closely similar to those of LiNCS. The coordination modes of SCN in dimers and in (LiNCS)4 are unusual for that ligand (nitrogen (μ2 and μ3). Ionic and organolithium aggregates are similar although organolithium compounds are even more associated and often give hexamers. Ionic aggregates intermingle with organolithium aggregates to give mixed species. Although underestimated for a long time the role that is played by all these species in organic chemistry is now more and more recognized. © 1990 De Gruyter