Most common oxidation state: +2
Density 9.91 g/cm3
Characteristics: Silvery-gray metal. Not oxidized by air under ordinary conditions. Easily dissolved in dilute nitric acid.
Characteristic reactions of Ni2+:
Nickel(II) ion forms a large variety of complex ions, such as the green hydrated ion, [Ni(H2O)6]2+.
Aqueous ammonia precipitates green gelatinous Ni(OH)2:
The nickel(II) hydroxide precipitate dissolves in excess ammonia to form a blue complex ion:
Sodium hydroxide also precipitates nickel(II) hydroxide:
Nickel(II) hydroxide does not dissolve in excess NaOH.
Addition of an alcoholic solution of dimethylglyoxime to an ammoniacal solution of Ni(II) gives a rose-red precipitate, abbreviated Ni(dmg)2:
Black NiS is precipitated by basic solutions containing sulfide ion:
Nickel(II) sulfide is not precipitated by adding H2S in an acidic solution. In spite of this, NiS is only slightly soluble in HCl and has to be dissolved in hot nitric acid or aqua regia, because NiS changes to a different crystalline form with different properties.
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