Page 83 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

COMPLEXONES 2.26

the complex. Thus the complexes formed by the nickel(I1) ion with (a) the
monodentate NH, molecule, (b) the bidentate ethylenediamine (1,2-diamino-
ethane), and (c) the hexadentate ligand 'penten' {(H ,N. CH ,.CH
CHz.CHz.N(CH z~CHz~NHz)z) show an overall stability constant value
for the ammonia complex of 3.1 x IO8, which is increased by a factor of about
10" for the complex of ligand (b), and is approximately ten times greater
still for the third complex.
The most common steric effect is that of inhibition of complex formation
owing to the presence of a large group either attached to, or in close proximity
to, the donor atom.
A further factor which must also be taken into consideration from the
point of view of the analytical applications of complexes and of complex-
formation reactions is the rate of reaction: to be analytically useful it is usually
required that the reaction be rapid. An important classification of complexes is
based upon the rate at which they undergo substitution reactions, and leads to
the two groups of labile and inert complexes. The term labile complex is
applied to those cases where nucleophilic substitution is complete within the
time required for mixing the reagents. Thus, for example, when excess of aqueous
ammonia is added to an aqueous solution of copper(I1) sulphate, the change
in colour from pale to deep blue is instantaneous; the rapid replacement of
water molecules by ammonia indicates that the Cu(I1) ion forms kinetically
labile complexes. The term inert is applied to those complexes which undergo
slow substitution reactions, i.e. reactions with half-times of the order of hours
or even days at room temperature. Thus the Cr(II1) ion forms kinetically inert
complexes, so that the replacement of water molecules coordinated to Cr(II1)
by other ligands is a very slow process at room temperature.
Kinetic inertness or lability is influenced by many factors, but the following
general observations form a convenient guide to the behaviour of the complexes
of various elements.
(i) Main group elements usually form labile complexes.
(ii) With the exception of Cr(II1) and Co(III), most first-row transition
elements form labile complexes.
(iii) Second- and third-row transition elements tend to form inert complexes.
For a full discussion of the topics introduced in this section a textbook of
inorganic chemistry (e.g. Ref. 4) or one dealing with complexes (e.g. Ref. 2),
should be consulted.

2.26 COMPLEXONES

The formation of a single complex species rather than the stepwise production
of such species will clearly simplify complexometric titrations and facilitate the
detection of end points. SchwarzenbachZ realised that the acetate ion is
able to form acetato complexes of low stability with nearly al1 polyvalent cations,
and that if this property could be reinforced by the chelate effect, then much
stronger complexes would be formed by most metal cations. He found that the
aminopolycarboxylic acids are excellent complexing agents: the most important
of these is 1,2-diaminoethanetetra-acetic acid (ethylenediaminetetra-acetic acid).
The formula (1) is preferred to (II), since it has been shown from measurements
of the dissociation constants that two hydrogen atoms are probably held in the
form of zwitterions. The values of pK are respectively pK, = 2.0, pKz = 2.7,