Dihydroxide

Dihydroxide is a term used in chemistry to denote a compound containing two hydroxide (OH−) groups. It is not a single, unique molecule, but a descriptor that appears in the systematic naming of metal hydroxides with the formula M(OH)2, where two hydroxide ligands are associated with a central metal atom or ion.

In inorganic nomenclature, dihydroxide commonly indicates two hydroxide ligands around a metal. For example, calcium dihydroxide corresponds to Ca(OH)2; magnesium dihydroxide corresponds to Mg(OH)2; iron(II) dihydroxide to Fe(OH)2; and nickel(II) dihydroxide to Ni(OH)2. In everyday usage, these compounds are more often referred to by their standard names: calcium hydroxide, magnesium hydroxide, iron(II) hydroxide, and nickel(II) hydroxide.

Properties and applications vary by metal. The solid hydroxides generally have limited solubility in water; Ca(OH)2 is sparingly soluble and is widely used in construction, lime production, and soil pH adjustment. Mg(OH)2 is only slightly soluble and is employed as an antacid and laxative. Iron(II) and nickel(II) hydroxides are less soluble and find roles in materials chemistry and catalysis, with solubility and stability depending on pH and oxidation state. Synthesis typically involves precipitation from solutions containing the metal ions by adding a base, or by direct reaction of metal oxides with water in some cases.

Terminology note: dihydroxide is a precise descriptor for M(OH)2 species, but in practice the common names or the formula M(OH)2 are more frequently used to avoid ambiguity.