State of the Ions in Aqueous Solution and Consequences

In water, ions are not naked. They are more or less energetically solvated by water molecules. Here the adj ective solvated must be taken to have its usual meaning. The solvation of ions implicates phenomena that may be a simple ion-dipole interaction between the ion and water as well as a true chemical bond between both, as is the case in aqua complexes. Whatever the case, the reactivity of ions in aqueous solutions is strongly influenced by these solvation phenomena. As a result, it is also the case of the formation of complexes starting from the central metallic ion. 

The ions of alkali metals, which are the most electropositive elements, generally exhibit the weakest tendency to form complexes. As an example, we can mention the fact that they give poorly stable complexes with EDTA, which is a powerful com-plexating agent of numerous metallic ions (see Chap. 28). (However, let us recall that these ions are complexed by crown ether compounds and by cryptands, but according to a particular mechanism.) In water, alkali ions must be considered as being only solvated by water in the restrictive meaning, that is, by ion-water dipole interactions for the first sheath of solvation and by water-dipoles interactions for the second. 

Cations of alkaline earth metals and those of lanthanides give few complexes but more than the preceding ones, although they are only a little less electropositive than them. In some conditions, they give complexes with EDTA whose stability is sufficient to receive analytical applications (see Chap. 28). 

Of course, other kinds of aqua complexes are encountered. For example, Co + yields both octahedral and tetrahedral aqua complexes. 

All these aqua complexes are nothing more than a particular case of coordination compounds, which we have already studied. From a structural standpoint, indeed, nothing fundamental distinguishes the complex [Cr (H20)6] + from the complex hexaammineehrome(III) [Cr (NH3)6] + or from the complex hexathiocyanatochro-mate(III) [Cr (NCS)6] , for example. 

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