Bioinorganic Chemistry
643
the ligand in selecting among metal ions.
Thus, ligands with oxygen donor atoms
are least discriminating among metal ions,
nitrogen donors intermediate, and sulfur
donors most discriminating.
Not only do Co
2
+
and Zn
2
+
display
nearly identical radii for the same co-
ordination number in Table 3 but they
also exhibit similar stabilities for all lig-
ands in Table 4, except for those involv-
ing sulfur and hydroxide. The similar-
ities allow the facile and useful sub-
stitution of Co
2
+
for Zn
2
+
in many
enzymes. When a sulfhydryl group is
present,
as
in
2-mercaptoethylamine,
Zn
2
+
binding strengthens, equaling that
for Ni
2
+
. Table 4 also shows the rel-
ative strengthening of Cd
2
+
and Pb
2
+
binding with the sulfhydryl donor in 2-
mercaptoethylamine.
To most unidentate ligands, Cd
2
+
binds
more strongly than Zn
2
+
. Probably be-
cause of an unfavorable ring bite size for
the relatively large Cd
2
+
(Table 3), Zn
2
+
chelates more strongly to ligands contain-
ing O and N donors. Table 2 shows that
upon introduction of an S donor atom
into a chelate, Cd
2
+
becomes the stronger
metal ion binder.
For all donor sets in Table 4, Hg
2
+
b
ind
ss
os
t
r
ong
l
yth
a
ti
ti
so
f
fth
eend
of the ruler scale. The number under Hg
in the last column of Table 4 refers to
there
la
t
ived
is
tancebywh
ichtheleng
th
of the whole log stability-constant scale
from Mg
2
+
to Cu
2
+
must be extended
to reach the value for Hg
2
+
. A telling
contrast appears between the length of the
extension for most bidentate compared to
unidentate ligands. The scale extension for
Hg
2
+
amounts to 0.2 to 0.4 log units for
bidentate ligands and to 1.1 to 1.2 log units
for the 3 unidentate ligands at the end
of Table 4. The difference arises because
Hg
2
+
prefers linear two-coordination and
binds the second donor atom in small
chelate rings much more weakly than the
±rst donor atom.
We may generalize the results and
conclusions of the stability ruler by noting
that alkali metal and alkaline earth metal
ions, lanthanides, and Al
3
+
prefer oxygen
donors; transition metal ions, oxygen and
nitrogen donors; and the heavy-metal ions,
nitrogen and sulfur donors.
7
Metal-ion Hydrolysis
The higher the charge density or charge-
to-radius ratio, the more likely it is for a
metal ion to undergo hydrolysis in aqueous
solutions to form hydroxo complexes.
Hydroxo complexes may abruptly form
polynuclear
complexes
and
precipitate
even in solutions more acidic than the
p
K
a
for ±rst hydroxo complex formation.
The ±rst ±ve small metal ions in Table 3
h
y
d
r
o
l
y
z
ee
v
e
ni
na
c
i
d
i
cs
o
l
u
t
i
o
n
sa
n
d
form precipitates. In six coordination, the
charge-to-radius ratio for the ±rst ±ve
metal ions is greater than 0.044, while
for all the other metal ions in Table 3,
the ratio is less than 0.035. The ±rst ±ve
metal ions cannot occur to an appreciable
extent in the bloodstream (pH 7.4) as the
free aqueous ion and must be complexed
in some way. Covalence may also promote
complex formation and hydrolysis in acidic
solutions, as is the case for Hg
2
+
.
Hydroxo complex formation follows the
same stability order as other ligands with
the strongest binding at the right of
the stability ruler in Table 4, where the
hydroxide ion appears as the ±rst entry.
The stability ruler in Table 4 shows that
Zn
2
+
and Pb
2
+
exhibit relatively strong
tendencies to form hydroxo complexes and
precipitates.
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