Page 81 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

FACTORS INRUENCINC THE STABlLlW OF COMPLEXES 2.25

In the above equilibria it has been assumed that no insoluble products are
formed nor any polynuclear species.
A knowledge of stability constant values is of considerable importance in
analytical chemistry, since they provide information about the concentrations
of the various complexes formed by a metal in specified equilibrium mixtures;
this is invaluable in the study of complexometry, and of various analytical
separation procedures such as solvent extraction, ion exchange, and chromato-
graphy.'v3

2.24 METAL ION BUFFERS
Consider the equation for complex formation
M + L = ML; K = [ML]/[M] [LI
and assume that ML is the only complex to be formed by the particular system.
The equilibrium constant expression can be rearranged to give:

log[M] = log 1/K +log[ML]/[L]
pM = log K - log[ML]/[L]

This shows that the pM value of the solution is fixed by the value of K and the
ratio of complex-ion concentration to that of the free ligand. If more of M is
added to the solution, more complex will be formed and the value of pM will
not change appreciably. Likewise, if M is removed from the solution by some
reaction, some of the complex will dissociate to restore the value of PM. This
recalls the behaviour of buffer solutions encountered with acids and bases
(Section 2.20), and by analogy, the complex-ligand system may be termed a
metal ion buffer.

2.25 FACTORS INFLUENCING THE STABlLlTY OF COMPLEXES
The stability of a complex will obviously be related to (a) the complexing ability
of the metal ion involved, and (b) characteristics of the ligand, and it is important
to examine these factors briefly.

(a) Complexing ability of metals. The relative complexing ability of metals is
conveniently described in terms of the Schwarzenbach classification, which is
broadly based upon the division of metals into Class A and Class B Lewis acids,
i.e. electron acceptors. Class A metals are distinguished by an order of affinity
(in aqueous solution) towards the halogens F- B Cl- >Br- > 1-, and form
their most stable complexes with the first member of each group of donor atoms
in the Periodic Table (i.e. nitrogen, oxygen and fluorine). Class B metals
coordinate much more readily with 1- than with F- in aqueous solution, and
form their most stable complexes with the second (or heavier) donor atom from
each group (i.e. P, S, Cl). The Schwarzenbach classification defines three
categories of metal ion acceptors:
1. Cations with noble gas configurations. The alkali metals, alkaline earths and
aluminium belong to this group which exhibit Class A acceptor properties.
Electrostatic forces predominate in complex formation, so interactions