Acidity or Hydrolysis Reactions

Definition: A Lewis acid is an electron pair acceptor

Definition: A Lewis base is an electron pair donor

In a complex, ligands donate a lone pair of electrons to the central metal ion

The ligands act as Lewis bases

The central metal ion act as a Lewis acid

In aqueous solutions, metal aqua ions are formed

The metal ions form hexaaqua complexes such as [M(H2O)6]2+ and [M(H2O)6]3+

These complexes have a octahedral shape with a co-ordination number of 6

Copper, cobalt and iron(II) form hexaaqua complexes in aqueous solution

Anhydrous white copper(II) sulfate dissolves in water to produce blue hexaaquacopper(II)

CuSO4 + 6H2O → [Cu(H2O)6]2+ + SO42-

White solid

Blue

Blue anhydrous cobalt(II) chloride dissolves in water to produce pink hexaaquacobalt(II)

CoCl2 + 6H2O → [Co(H2O)6]2+ + 2Cl-

Blue solid

Pink

Pale green iron(II) sulfate dissolves in water to produce pale green hexaaquairon(II)

FeSO4 + 6H2O → [Fe(H2O)6]2+ + SO42-

Pale green solid

Pale green

Aluminium is not a transition metal Aluminium forms a Al3+ ion

Aluminium chloride dissolves in water to produce hexaaquaaluminium(III)

Hexaaquaaluminium(III) is colourless as it is not a transition metal and therefore it has no d to d transitions

AlCl3 + 6H2O → [Al(H2O)6]3+ + 3Cl-

Colourless

Chromium(III) compounds such as chromium chloride dissolve in water to produce ruby hexaaquachromium(III)

CrCl3 + 6H2O → [Cr(H2O)6]3+ + 3Cl-

Ruby

Iron(III) compounds such as iron(III) chloride dissolve in water to produce pale violet hexaaquairon(III)

FeCl3 + 6H2O → [Fe(H2O)6]3+ + 3Cl-

Pale violet

Summary of hexaaqua colours:

Oxidation State Complex Colour+2 [Cu(H2O)6]2+ Blue

+2 [Co(H2O)6]2+ Pink

+2 [Fe(H2O)6]2+ Pale green

+3 [Al(H2O)6]3+ Colourless

+3 [Cr(H2O)6]3+ Ruby

+3 [Fe(H2O)6]3+ Pale violet