Chemistry Reference
In-Depth Information
TABLE 3.8
Overall Stability Constants for Some Complexes with
Mono- and Bidentate Ligands in Aqueous Solution at 298 K
Complex ML
n
β
n
= [ML
n
]/[M][L]
n
Complex ML
n
β
n
= [ML
n
]/[M][L]
n
L = NH
3
L = H
2
NCH
2
CH
2
NH
2
Mn(NH
3
)
6
2+
20
Mn(en)
3
2+
5∙10
5
Fe(NH
3
)
6
2+
1.6∙10
2
Fe(en)
3
2+
4∙10
9
Co(NH
3
)
6
2+
1.3∙10
5
Co(en)
3
2+
8∙10
13
Ni(NH
3
)
6
2+
5.5∙10
8
Ni(en)
3
2+
4∙10
18
Cu(NH
3
)
4
2+
1∙10
12
Cu(en)
2
2+
1.6∙10
20
Zn(NH
3
)
4
2+
5∙10
8
Zn(en)
2
2+
1.2∙10
13
The value of an overall stability constant β
n
is the product of the stepwise stability
constants K
1
to K
n
:
n
β=Π
K
(3.53)
n
n
1
The magnitude of log
K
reflects the tendency toward formation of metal com-
plexes in aqueous solution and gives a quantitative measure of the relative stabilities
of metal complexes. The larger the magnitude of the equilibrium constant, the more
stable the complex ML is in solution.
The stability is greatly increased in the case of complexes formed by the multi-
dentate ligands. The bi- or polydentate ligands form coordination compounds in the
shape of a heteroatomic ring. This process of ring formation is called
chelation
, the
ligands involved are called chelating agents, and the resulting complex is called a
chelate
. The increase in stability is due to a predominantly entropic effect. Table 3.8
presents the increase of overall stability constants of ethylenediamine chelates of
some divalent metal ions compared to the corresponding ammonia complexes.
Considering that each metal possesses its own spectra of affinity constants for
different biological ligands, the values of stability constants have been used as
descriptors for QSAR studies. Some examples include the log of the equilibrium
constant (log K
eq
) of a metal-ATP complex (Biesinger and Christensen 1972), the
stability constant of metal-ion complexes with NH
3
, the stability constants of metal
ion complexes with EDTA, and the stability constants of divalent metal ions with
AMP (Enache et al. 2003).
3.3.5 K
inEtic
r
Eactivity
/l
ability
of
m
Etal
c
omplExEs
The coordination complexes that undergo rapid ligand exchange reactions are
referred to as
labile complexes
; those that do not are called
inert complexes
. Metal
complexes vary in lability from extremely labile to essentially substitutionally inert.
The intrinsic nature of the metal ion largely determines the reactivity of the metal
complex. In this respect, the rates of replacement of ligand of aqua complexes by
