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Concept
Olation
Résumé
In inorganic chemistry, olation is the process by which metal ions form polymeric oxides in aqueous solution. The phenomenon is important for understanding the relationship between metal aquo complexes and metal oxides, which are represented by many minerals.
At low pH, many metal ions exist in aqueous solution as aqua coordination complexes, often with the formula [M(H2O)6]3+. As the pH increases, one O-H bond ionizes to give the hydroxide complex, the conjugate base of the parent hexaaqua complex:
[M(H2O)6]3+ [M(H2O)5OH]2+ + H+
The hydroxo complex is poised to undergo olation, which is initiated by displacement of one water by a neighboring complex:
[M(H2O)5OH]2+ + [M(H2O)6]3+ M(H2O)5(μ-OH)M(H2O)5}5+ + H2O
In this product, the hydroxide ligand bridges between the two metals, this bridge is denoted with the symbol μ.
In the resulting 5+ ion, the remaining water and hydroxo ligands are highly acidic and the ionization and condensation processes can continue at still higher pHs. The formation of the oxo-dimer is a process called "oxolation," although sometimes olation and oxolation are not distinguished:
{[M(H2O)5]2(μ-OH)}5+ {[M(H2O)5]2(μ-O)}4+ + H+
Ultimately one observes the formation of the metal oxide:
2 [M(H2O)6]3+ M2O3 + 9 H2O + 6 H+
Olation and oxolation are responsible for the formation of many natural and synthetic materials. Such materials are usually insoluble polymers, but some, the polyoxometallates, are discrete and molecular.
One application where olation is important is leather tanning using chromium(III) sulfate. This salt dissolves to give hexaaquachromium(III) cation, [Cr(H2O)6]3+ and sulfate anions. [Cr(H2O)6]3+ acts as an acid according to the reaction:
[Cr(H2O)6]3+ [Cr(H2O)5OH]2+ + H+; Keq ~ 10−4 M
Thus, higher pH favors [Cr(H2O)5OH]2+. This hydroxy complex can undergo olation:
[Cr(H2O)6]3+ + [Cr(H2O)5OH]2+ → [(Cr(H2O)5)2(μ-OH)]5+ + H2O
2[Cr(H2O)5OH]2+ → [(Cr(H2O)4)2(μ-OH)2]4+ + 2 H2O
The "diol" (second reaction) is favored and is accelerated by heat and high pH.
Source officielle
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