Olation
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In inorganic chemistry, olation is the process by which metal ions form polymeric oxides in aqueous solution.[1] The phenomenon is important for understanding the relationship between metal aquo complexes and metal oxides, which are represented by many minerals.[2]
At low pH, many metal ions exist in aqueous solution as aquo complexes with the formula [M(H2O)6]3+.[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 the hydroxide ligand on another complex:
- [M(H2O)6]3+ + [M(H2O)5OH]2+ ⇌ [M2(H2O)10(μ−OH]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:[4]
- 2 [LnMOH] ⇌ LnM−O−MLn + H2O, where L = ligand
Ultimately olation and oxolation lead to metal oxides:
- 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.