Olation
Encyclopedia
In inorganic chemistry
, olation is the process by which metal ions form polymeric oxide
s in aqueous solution. The phenomenon is important to understand the relationship between metal ions in aqueous solution and metal oxides, which are represented by many mineral
s.
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 water ligand
ionizes to give the hydroxide
complex, the conjugate base of the parent hexaaqua complex
:
The hydroxo complex is poised to undergo olation, which is initiated by displacement of one water by from a neighboring complex:
In the resulting 5+ ion, the remaining water and hydroxo ligands are highly acidic and the ionization and condensation
processes can continue. The formation of the oxo-dimer is a process called “oxolation,” although sometimes olation and oxolation on not distinguished:
Ultimately one observes the formation of the metal oxide:
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.
Thus, higher pH favors [Cr(H2O)5OH]2+. This hydroxy complex can undergoes olation:
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)2]4+. The chromium(III) hydroxide is susceptible to oxolation:
Products of oxolation are less susceptible to acidic cleavage than the hydroxy bridge. The resulting clusters are the active in crosslinking the protein in tanning. which essentially involves the cross-linking of the collagen subunits. The actual chemistry of [Cr(H2O)6]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
s, as well as "masking agents." Masking agents are carboxylic acid
s, 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.
Inorganic chemistry
Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry...
, olation is the process by which metal ions form polymeric oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s in aqueous solution. The phenomenon is important to understand the relationship between metal ions in aqueous solution and metal oxides, which are represented by many mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s.
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 water ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
ionizes to give the hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
complex, the conjugate base of the parent hexaaqua complex
Metal ions in aqueous solution
A metal ion in aqueous solution is a cation, dissolved in water, of chemical formula [Mn]z+. The solvation number, n, determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for elements in rows 3 and 4 of the periodic table. Lanthanide and actinide aqua ions have solvation...
:
- [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 from a neighboring complex:
- [M(H2O)5OH]2+ + [M(H2O)6]3+
{[M(H2O)5]2(μ-OH)}5+ + H2O
In the resulting 5+ ion, the remaining water and hydroxo ligands are highly acidic and the ionization and condensation
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
processes can continue. The formation of the oxo-dimer is a process called “oxolation,” although sometimes olation and oxolation on 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.
Olation and leather tanning
One application where olation is clearly 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 undergoes 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. 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)2]4+. The chromium(III) hydroxide is susceptible to oxolation:
- [(Cr(H2O)4)2(μ-OH)2]4+ → [(Cr(H2O)4)2(μ-O)2]2+ + 2 H+
Products of oxolation are less susceptible to acidic cleavage than the hydroxy bridge. The resulting clusters are the active in crosslinking the protein in tanning. which essentially involves the cross-linking of the collagen subunits. The actual chemistry of [Cr(H2O)6]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 acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s, as well as "masking agents." Masking agents are carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s, such as acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
, 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.