In
inorganic chemistry
Inorganic chemistry deals with synthesis and behavior of inorganic and organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subjects of organic chemistry. The distinction between the two disci ...
, olation is the process by which metal ions form polymeric
oxide
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
s 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
(H2O)6sup>3+. As the pH increases, one O-H bond
ionizes to give the
hydroxide 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 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
Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...
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
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,
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 acids, as well as "masking agents." Masking agents are
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
s, such as
acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...
, 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