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Nickel(II), Ni2+

Some green nickel(II) sulphate-6-water crystals

The hexaquo nickel(II) ion, [Ni(H2O)6]2+, is green in aqueous solution.

On addition of sodium hydroxide solution a green precipitate of nickel(II) hydroxide is formed. This can be represented as a simple precipitation reaction or as the deprotonation of the original hexaquo complex:

Green precipitate of nickel(II) hydroxide

Ni2+(aq) + 2OH-(aq) ⇒ Ni(OH)2(s)

or [Ni(H2O)6]2+(aq) + 2OH-(aq) ⇒ Ni(H2O)4(OH)2(s) + 2H2O(l)

This precipitate is insoluble in excess sodium hydroxide solution.

Violet hexammino nickel(II) complex

The same precipitate forms on addition of ammonia solution (ammonia solution is alkaline as it produces hydroxide ions in aqueous solution). However, in excess ammonia it dissolves due to the formation of the violet hexammino nickel(II) ion by ligand exchange.

Ni(H2O)4(OH)2(s) + 6NH3(aq) ⇒ [Ni(NH3)6]2+(aq) + 4H2O(l) + 2OH-(aq)

A test for nickel(II) ions involves the addition of a solution of dimethylglyoxime (butanedione dioxime). A positive test gives a bright red precipitate.

The formation of a bright red precipitate with dimethylglyoxime is used as a test for nickel(II)

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