Manganese(II) hydroxide is the inorganic compound with the formula Mn(OH)<sub>2</sub>. It is a white solid although samples darken quickly upon exposure to air owing to oxidation. It is poorly soluble in water.
Mn(OH)<sub>2</sub> adopts the brucite structure, i.e. the arrangement of the atoms in the crystal are the same as the arrangement of the atoms in Mg(OH)<sub>2</sub>. The Mn(II) centers are bonded to six hydroxide ligands. Each hydroxide ligand bridges to three Mn(II) sites. The O-H bonds are perpendicular to the planes defined by the oxygen atoms, projecting above and below these layers.
Manganese(II) hydroxide precipitates as a solid when an alkali metal hydroxide is added to an aqueous solution of Mn<sup>2+</sup> salt:
Manganese(II) hydroxide oxidises readily in air, as indicated by darkening of samples.
The compound adopts the brucite structure, as do several other metal dihydroxides.