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Shriver & Atkins: Inorganic Chemistry 4e

Chapter 19

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) two-way arrow NH4+(aq) + OH-(aq)

Ni2+(aq) + 2OH-(aq) two-way arrow 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) two-way arrow NH4+ (aq) + OH-(aq)

Zn2+(aq) + 2OH2+(aq) two-way arrow 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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