Condensation of metal complexes in solution forms entities in which the cations are linked by hydroxo (HO−) or oxo (O2−) bridges. The reaction is initiated by the addition of a base to an aquocomplex: . . . 2[Cr(OH2)6]3++ 2HO- → [Cr2(OH)2(OH2)8]4+ + 2 H2O . . . or by the addition of an acid to an anionic complex: . . . 2 [CrO4]2- + 2H+ → [Cr2O7]2- + H2O . . . Thus, purely aquo- and purely oxocomplexes are stable in solution, and the condensation of cations is initiated by hydroxylation. With regard to electrically charged hydroxylated complexes, the reaction forms discrete and soluble entities—polycations and polyanions with a molecular complexity which depends on acidity conditions. This chapter presents a detailed study of their formation and structure. With regard to noncharged hydroxylated complexes, the condensation reaction is no longer limited and leads to the formation of a solid (a subject that is examined in the following chapters). The hydroxylation reaction is the key stage to initiate the condensation of cations in solution. It is thus important to precise the mechanism of the successive steps of the process, in order to understand why the behavior of a cation is closely related to its oxidation state, and why the reaction product may be a discrete molecular species or a solid. As a cation generally exhibits its maximum coordination number in the initial monomeric complex and in condensed species, the condensation reaction is a substitution that proceeds according to one of three basic mechanisms: dissociation, association, and interchange or direct displacement [1, 2]. Dissociative substitution is a two-step process involving the formation of a reduced-coordination intermediate: In the first step, a labile ligand, the leaving group, breaks its bond in the starting complex before a nucleophilic entering group completes, in the second step, the cation coordination (Fig. 3.1 a). Associative substitution is also a two-step process in which the intermediate temporarily has increased coordination. The bond with the nucleophilic entering group (first step) occurs prior to the release of the leaving group (second step) (Fig. 3.1 b).
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