Olation
At low pH, many metal ions exist in aqueous solution as aqua coordination complexes, often with the formula [M(HO)]. As the pH increases, one O-H bond 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 a neighboring complex:
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," although sometimes olation and oxolation are not distinguished:
Ultimately one observes the formation of the metal oxide:
Nickel compounds
Nickel hydroxides are used in nickel–cadmium and Nickel–metal hydride batteries.
Nickel(II) hydroxide Ni(OH), the main hydroxide of nickel is coloured apple green. It is known as the mineral theophrastite.
β-NiO(OH) is a black powder with nickel in the +3 oxidation state. It can be made by oxidising nickel nitrate in a cold alkaline solution with bromine. A mixed oxidation state hydroxide NiO(OH) is made if oxidation happens in a hot alkaline solution. A Ni hydroxide: nickel peroxide hydrate NiO, can be made by oxidising with alkaline peroxide. It is black, and unstable and oxidises water.