Colorful complexes of ammonia
Transition metal complexes are often characterized by their colors.

Hexaaquacopper(II) ion is pale blue while the hexaamminecopper(II) ion is a intense blue color.

Hexaaquanickel(II) ion is pale green while the hexaamminenickel(II) ion is a deep blue color.

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Complexation of iron(III) ion with chloride ion
Dissolving a hydrated iron(III) salt in nitric acid results in the formation of the very pale purple (essentially colorless) hexaaquairon(III) ion, [Fe(OH₂)₆]³⁺(aq) . However, if hydrochloric acid is used, the yellow tetrachloroiron(III) ion, [FeCl₄]^-(aq) , is formed.

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A specific test for iron(III) ion I
Iron(III) ion reacts with the hexacyanoferrate(II) ion to give an intense blue precipitate of Prussian Blue:

4Fe³⁺(aq) + 3[Fe(CN)₆]^4-(aq) → Fe₄[Fe(CN)₆]₃(s)

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A specific test for iron(III) ion II
Iron(III) ion reacts with thiocyanate ion to give a deep red color of the thiocyanato complex:

[Fe(OH₂)₆]³⁺(aq) + SCN^-(aq) → [Fe(SCN)(OH₂)₅]²⁺(aq) + H₂O(l)

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A specific test for iron(II) ion
Iron(II) ion reacts with the hexacyanoferrate(III) ion to give the intense blue precipitate of Prussian Blue.

3Fe²⁺ (aq) + 4[Fe(CN)₆]^3-(aq) → Fe₄[Fe(CN)₆]₃(s) + 6CN^-(aq)

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Complexation of the nickel(II) ion
Addition of ammonia to the green hexaaquanickel(II) ion results in the formation of the blue hexaamminenickel(II) ion.

[Ni(OH₂)₆]²⁺(aq) + 6NH₃(aq) → [Ni(NH₃)₆]²⁺(aq) + 6H₂O(l)

At the interface, a pale blue precipitate can be seen. This is a result of the reaction between the nickel(II) ion and the hydroxide ion from the hydrolysis of the ammonia (i.e. the solution is basic):

NH₃(aq) + H₂O(l) NH₄⁺(aq) + OH^-(aq)

Ni²⁺(aq) + 2OH^-(aq) Ni(OH)₂(s)

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Colorful copper(II) complexes
Addition of excess chloride ion to the pale blue hexaaquacopper(II) ion gives the green tetrachlorocuprate(II) ion.

[Cu(OH₂)₆]²⁺(aq) + 4Cl^-(aq) → [CuCl₄]^2- (aq) + 6H₂O(l)

Addition of aqueous ammonia (which has a lower density than water) gives a deep blue layer of the hexaamminecopper(II) complex.

[CuCl₄]^2-(aq) + 6NH₃(aq) → [Cu(OH₂)₆]²⁺(aq) + 4Cl^-(aq)

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Complexation of zinc ion with ammonia
When zinc ion reacts with ammonia, the first product is zinc hydroxide (compare with "Complexation of the nickel(II) ion" above):

NH₃(aq) + H₂O(l) NH₄⁺ (aq) + OH^-(aq)

Zn²⁺(aq) + 2OH²⁺(aq) Zn(OH)₂(s)

With excess ammonia solution, the hexaamminezinc ion is formed.

Zn(OH)₂(s) + 6NH₃(aq) → [Zn(NH₃)₆]²⁺(aq) + 2OH^-(aq)

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