Lewis acids polyvalent

Positive ions are often regarded as Lewis acids. However, many of these, particularly polyvalent ions, are strongly hydrated in solution and thus become ordinary Bronsted acids (of the same type as Co(NH3)q+). Thus salts of tripositive iron yield a hydrate, expressed noncommittally as Fe(H20)l+ whose dissociation constant in water has been found to be about the same as that of H3PO4 ... 

The simplest metal-ligand bond is perhaps L M-NH3, where an ammonia binds to a metal fragment. This fragment will nsnally have other ligands, represented here by L . The bond consists of the lone pairs of electrons present in free NH3 that are donated to the metal to form the complex. The metal is a polyvalent Lewis acid capable of accepting the lone pairs of several ligands, L, which act as Lewis bases. 

A variety of organic compounds are oxidized by salts of polyvalent metals. We have already discussed one-electron oxidations by some Lewis acids and noted that the mechanisms of these oxidations are unknown. In contrast, oxidations by some other metal salts have been studied quite thoroughly and much is known about the mechanisms. Of particular interest to us are cobalt(III), man-aganese(III), lead(IV), and cerium(IV) salts, and the most commonly used are the acetates. Another contrast with the Lewis acids is that cation radicals formed in oxidations by salts of polyvalent metals usually have short lifetimes since they are involved in further... 

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