View source for Chemistry:Olation

In [[Chemistry:Inorganic chemistry|inorganic chemistry]], '''olation''' is the process by which metal ions form polymeric [[Chemistry:Oxide|oxide]]s in aqueous solution.<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. {{ISBN|0-12-352651-5}}.</ref>  The phenomenon is important for understanding the relationship between [[Chemistry:Metal aquo complex|metal aquo complex]]es and metal oxides, which are represented by many [[Earth:Mineral|mineral]]s.
At low pH, many metal ions exist in aqueous solution as aqua [[Chemistry:Coordination complex|coordination complex]]es, often with the formula [M(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup>.<ref>Helm, L. and Merbach, A. E., "Inorganic and Bioinorganic Solvent Exchange Mechanisms", Chem. Rev., 2005, 105, 1923-1959 {{doi|10.1021/cr030726o}}</ref>  As the pH increases, one O-H bond ionizes to give the [[Chemistry:Hydroxide|hydroxide]] complex, the conjugate base of the parent [[Chemistry:Metal ions in aqueous solution|hexaaqua complex]]:

:[M(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> {{eqm}} [M(H<sub>2</sub>O)<sub>5</sub>OH]<sup>2+</sup> + H<sup>+</sup>

The hydroxo complex is poised to undergo olation, which is initiated by displacement of one water by a neighboring complex:

:[M(H<sub>2</sub>O)<sub>5</sub>OH]<sup>2+</sup> + [M(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> {{eqm}} M(H<sub>2</sub>O)<sub>5</sub>(μ-OH)M(H<sub>2</sub>O)<sub>5</sub>}<sup>5+</sup> + H<sub>2</sub>O

In this product, the hydroxide ligand [[Chemistry:Bridging 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 [[Physics:Condensation|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(H<sub>2</sub>O)<sub>5</sub>]<sub>2</sub>(μ-OH)}<sup>5+</sup> {{eqm}} {[M(H<sub>2</sub>O)<sub>5</sub>]<sub>2</sub>(μ-O)}<sup>4+</sup> +  H<sup>+</sup>

Ultimately one observes the formation of the metal oxide:

:2 [M(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> {{eqm}} M<sub>2</sub>O<sub>3</sub> + 9 H<sub>2</sub>O + 6 H<sup>+</sup>

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.

==Olation and leather tanning==
One application where olation is important is leather tanning using chromium(III) sulfate.  This salt dissolves to give hexaaquachromium(III) cation, [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> and sulfate anions. [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> acts as an acid according to the reaction:<ref>Schlesinger, M.; Paunovic, M.; Modern Electroplating. Wiley-Interscience, 4th ed. 2000, 209-212.</ref>
:[Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> {{eqm}} [Cr(H<sub>2</sub>O)<sub>5</sub>OH]<sup>2+</sup> + H<sup>+</sup>;  K<sub>eq</sub> ~ 10<sup>−4</sup> M  
Thus, higher pH favors [Cr(H<sub>2</sub>O)<sub>5</sub>OH]<sup>2+</sup>.  This hydroxy complex can undergo olation:<ref>M. Thompson, R. E. Connick "Hydrolytic polymerization of chromium(III). 1. Two dimeric species" Inorg. Chem. 1981, 20, 2279-2285. {{doi|10.1021/ic50221a068}}</ref>
:[Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> + [Cr(H<sub>2</sub>O)<sub>5</sub>OH]<sup>2+</sup> → [(Cr(H<sub>2</sub>O)<sub>5</sub>)<sub>2</sub>(μ-OH)]<sup>5+</sup> + H<sub>2</sub>O
:2[Cr(H<sub>2</sub>O)<sub>5</sub>OH]<sup>2+</sup> → [(Cr(H<sub>2</sub>O)<sub>4</sub>)<sub>2</sub>(μ-OH)<sub>2</sub>]<sup>4+</sup>  +  2 H<sub>2</sub>O
The "diol" (second reaction) is favored and is accelerated by heat and high pH.   The balance of these two factors, temperature and pH of the solution, along with the concentration of chromium(III), influence the continued polymerization of [(Cr(H<sub>2</sub>O)<sub>4</sub>)<sub>2</sub>(μ-OH)<sub>2</sub>]<sup>4+</sup>.<ref>Schlesinger, M.; Paunovic, M.; Modern Electroplating. Wiley-Interscience, 4th ed. 2000, 209-212.</ref>  The chromium(III) hydroxide is susceptible to oxolation:	
:[(Cr(H<sub>2</sub>O)<sub>4</sub>)<sub>2</sub>(μ-OH)<sub>2</sub>]<sup>4+</sup> → [(Cr(H<sub>2</sub>O)<sub>4</sub>)<sub>2</sub>(μ-O)<sub>2</sub>]<sup>2+</sup> + 2 H<sup>+</sup>
Products of oxolation are less susceptible to acidic cleavage than the hydroxy bridge. The resulting clusters are  active in crosslinking the protein in tanning.<ref>Harlan, J.; Feairheller, S.; Adv. Exp. Med. Biol. 1977, 86A, 425.</ref>  which essentially involves the cross-linking  of the collagen subunits. The actual chemistry  of [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> is more complex in the tanning bath rather than in water due to the presence of a variety of ligands. Some ligands include the sulfate anion, the collagen's carboxyl groups, amine groups from the side chains of the [[Chemistry:Amino acid|amino acid]]s, as well as "masking agents."  Masking agents are [[Chemistry:Carboxylic acid|carboxylic acid]]s, such as [[Chemistry:Acetic acid|acetic acid]], used to suppress formation of  polychromium(III) chains.  Masking agents allow the tanner to further increase the pH to increase collagen's reactivity without inhibiting the penetration of the chromium(III) complexes.  The crosslinks formed by the polychromium species are approximately 17 Å long.<ref>Gustavson, K.H.; The Chemistry of Tanning Processes.  Academic Press Inc., New York, 1956.</ref><ref>Covington, A.; Modern Tanning Chemistry, Chem. Soc. Rev. 1997, 26, 111-126. {{doi|10.1039/CS9972600111}}</ref>

==References==
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[[Category:Oxides| ]]

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