Page 79 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

COMPLEXATION 2.22

ions are present in solution. The dissociation of the complexion is represented by:

and the dissociation constant is given by:

The stability constant K = l/Kdiss, = 1.5 x IO7 mol-2 L2
The magnitude of the dissociation constant clearly shows that only a very small
silver ion concentration is ~roduced bv the dissociation of the com~lex ion.
The stability of complex ions varies within very wide limits. It is quantitatively
expressed by means of the stability constant. The more stable the complex, the
greater is the stability constant, i.e. the smaller is the tendency of the complex
ion to dissociate into its constituent ions. When the complex ion is very stable,
e.g. the hexacyanoferrate(I1) ion [Fe(CN),I4-, the ordinary ionic reactions of
the comDonents are not shown.
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The application of complex-ion formation in chemical separations depends
upon the fact that one component may be transformed into a complex ion which
no longer reacts with a given reagent, whereas another component does react.
One exam~le mav be mentioned here. This is concerned with the se~aration of
cadmium and copper. Excess of potassium cyanide solution is added to the
solution containing the two salts when the complex ions [Cd(CN),I2- and
[Cu(CN),I3- respectively are formed. Upon passing hydrogen sulphide into
the solution containing excess of CN- ions, a precipitate of cadmium sulphide
is produced. Despite the higher solubility product of CdS (1.4 x mol2 L-2
as against 6.5 x mol2 L-2 for copper(I1) sulphide), the former is
precipitated because the complex cyanocuprate(1) ion has a greater stability
constant (2 x mol-4 L4 as compared with 7 x 101° mol-4 L4 for the
cadmium compound).

2.22 COMPLEXATION
The processes of complex-ion formation referred to above can be described by
the general term complexation. A complexation reaction with a metal ion
involves the replacement of one or more of the coordinated solvent molecules
by other nucleophilic groups. The groups bound to the central ion are called
ligands and in aqueous solution the reaction can be represented by the
equation:

Here the ligand (L) can be either a neutral molecule or a charged ion, and
successive replacement of water molecules by other ligand groups can occur
until the complex ML, is formed; n is the coordination number of the metal
ion and represents the maximum number of monodentate ligands that can be
bound to it.
Ligands may be conveniently classified on the basis of the number of points
of attachment to the metal ion. Thus simple ligands, such as halide ions or the
molecules H20 or NH,, are monodentate, i.e. the ligand is bound to the metal
ion at only one point by the donation of a lone pair of electrons to the metal.