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 complex
In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorat ...
es and metal oxides, which are represented by many
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
s.
At low pH, many metal ions exist in aqueous solution as aqua
coordination complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as '' ligands'' or complexing agents. ...
es, often with the formula
(H2O)6sup>3+. As the pH increases, one O-H bond
ionize
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
s to give the
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...
complex, the conjugate base of the parent
hexaaqua complex:
:
(H2O)6sup>3+
(H2O)5OHsup>2+ + H
+
The hydroxo complex is poised to undergo olation, which is initiated by displacement of one water by a neighboring complex:
:
(H2O)5OHsup>2+ +
(H2O)6sup>3+ M(H
2O)
5(μ-OH)M(H
2O)
5}
5+ + H
2O
In this product, the hydroxide ligand
bridges
A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually someth ...
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:
:
5+ 4+ + H
+
Ultimately one observes the formation of the metal oxide:
:2
(H2O)6sup>3+ M
2O
3 + 9 H
2O + 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
polyoxometallate
In chemistry, a polyoxometalate (abbreviated POM) is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are ...
s, are discrete and molecular.
Olation and leather tanning
One application where olation is important is
leather tanning
Tanning is the process of treating skins and hides of animals to produce leather. A tannery is the place where the skins are processed.
Tanning hide into leather involves a process which permanently alters the protein structure of skin, makin ...
using chromium(III) sulfate. This salt dissolves to give hexaaquachromium(III) cation,
r(H2O)6sup>3+ and sulfate anions.
r(H2O)6sup>3+ acts as an acid according to the reaction:
:
r(H2O)6sup>3+
r(H2O)5OHsup>2+ + H
+; K
eq ~ 10
−4 M
Thus, higher pH favors
r(H2O)5OHsup>2+. This hydroxy complex can undergo olation:
:
r(H2O)6sup>3+ +
r(H2O)5OHsup>2+ →
Cr(H2O)5)2(μ-OH)sup>5+ + H
2O
:2
r(H2O)5OHsup>2+ →
Cr(H2O)4)2(μ-OH)2sup>4+ + 2 H
2O
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(H2O)4)2(μ-OH)2sup>4+. The chromium(III) hydroxide is susceptible to oxolation:
:
Cr(H2O)4)2(μ-OH)2sup>4+ →
Cr(H2O)4)2(μ-O)2sup>2+ + 2 H
+
Products of oxolation are less susceptible to acidic cleavage than the hydroxy bridge. The resulting clusters are active in crosslinking the protein in tanning. which essentially involves the cross-linking of the collagen subunits. The actual chemistry of
r(H2O)6sup>3+ 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
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
s, as well as "masking agents." Masking agents are
carboxylic acids, such as
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.
[Covington, A.; Modern Tanning Chemistry, Chem. Soc. Rev. 1997, 26, 111-126. {{doi, 10.1039/CS9972600111]
References