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Complexation Reactions

Introduction

Consider the example of adding Cl- to a solution containing Ag+. As you might predict, a precipitate forms because AgCl is an insoluble salt. What happens if we continue adding Cl- to the solution containing the AgCl precipitate? Considering only Ksp and the common-ion effect, we expect the solubility of AgCl to decrease continuously as the concentration of Cl- increases. The solubility of AgCl decreases as the concentration of Cl- increases to 5x10-3 M where it reaches a minimum. As [Cl-] increases above 5x10-3 M, the solubility of AgCl increases due to the formation of AgCl2- and AgCl32-. Such a compound is called a complex, and can increase the solubility of an insoluble salt as a complex forms.


Nomenclature

Ligand
Neutral or anionic species with unpaired electrons that can bond to a metal ion. Common ligands are CN-, NH3, OH-, and halides.
Complex
An association of a central metal ion and surrounding ligands in solution - also called coordination complex.
Coordination number
The number of bonds with the central metal ion, usually 2, 4, or 6.
Chelate
A specific type of complex in which at least one ligand contains more than one atom with unpaired electrons so it can make multiple bonds with the central metal ion. Ligands that make two bonds are called bidendate, ligand that make three bonds are tridendate, and so on.

Examples

Metals with oxidation states of +1, +2, +3, or +4 can exist as complexes in water. Although we discuss the metal concentration in solution, the metal really exists as an aquo complex.

Note that metals can exist in higher oxidation states, but the charge density is so high that they do not exist as complexes in solution. They form oxyanions such as:

As an example, what is present when Cu(NO3)2 is added to water?

We have Cu2+ and NO3- in solution. The NO3- is a spectator ion and we don't expect it to take part in any equilibrium. Although we discuss [Cu2+], the Cu2+ probably exists as a tetra-aquo complex Cu(H2O)42+. Water has two lone pairs of electrons on the oxygen and can form complexes with transition metal ions. We can think of complexes in solution being in competitive equilibria with other species. If NH3 is added, a Cu(NH3)42+ complex forms because the NH3 complexes with Cu2+ more strongly than does water.

Ru(bpy)32+
(tris-bpy)Ru
tris(bipyridyl)ruthenium consists of a central ruthenium (green) surrounded by three bipyridyl ligands. The nitrogen atoms (blue) have lone pairs of electrons to donate to empty orbitals on the ruthenium to form the complex bonds. Stable complexes form for the ruthenium ion in both the 2+ and 3+ oxidation states. The charge of the central ion is not as critical as the overlap of lone pairs of electrons with empty orbitals on the metal for the stability of complexes.


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