646
Bioinorganic Chemistry
to oxygen or sulfhydryl donor atoms.
Stability constants have been tabulated
for binding to substituted iminodiacetates,
R–N(CH
2
COO
)
2
.ForR
=−
CH
2
COO
(nitrilotriacetate) and R
=−
CH
2
CH
2
S
,
the difference in stability-constant logs
for the former minus the latter tridentate
ligand
is
given
in
parentheses,
in
sequence, after each dipositive metal ion.
Ca(1.5), Sr(1.4), Ba(1.3), Mg(1.1)
À
Mn(
1.9),
Ni
(
2.2),
Fe
(
2.9)
À
Zn
(
5.3),
Pb
(
5.6),
Cd
(
6.9),
Hg
(
<
8).
O–S scale
Similar differences and sequences are
obtained with R
=−
CH
2
CH
2
OH as the
oxygen donor, with the addition of poorly
chelating Hg
2
+
, still with the greatest
negative value. Metal ions in the O–S scale
fall distinctly into three groups: alkaline
earths with differences from
+
1.5 to
+
1.1,
transition metal ions from
1.9 to
2.9,
and ±lled d-orbital ions from
5.3 to
<
8.
The spans within the ±rst two groups are
narrower than the gaps between the three
groups. For oxygen–sulfhydryl binding,
the alkaline earths may be designated as
hard, the three transition metal ions as
borderline, and the four ±lled d-shell metal
ions as soft. The recognized avidity of
Zn
2
+
for sulfhydryl donors is con±rmed.
Though assigned as hard by Pearson,
by this grouping Mn
2
+
is emphatically
borderline. This borderline designation
for Mn
2
+
is also more consistent with
other scales.
8.4
Ligand Scale
We now create a ligand scale, correspond-
ing to the second substitution reaction of
Sect. 8.2, by considering the differences
between stability-constant logarithms for
soft and hard metal ions for several lig-
ands. Hg
2
+
stands as the preeminent
soft metal ion. Sc
3
+
is the hardest metal
ion in several scales, but results are lim-
ited. More ligands may be included if
results for the smaller and heavier lan-
thanides are considered for the hard metal
ion. For 16 ligands, the difference be-
tween the stability-constant logarithms for
Hg
2
+
minus a lanthanide in parentheses
is as follows:
F
(
3), H
2
O(0), acetate(2),
HCO
3
(4), CO
3
2
(5),
OH
(
5
)>
pyridine
(6),
benzimidazole
(6),
imidazole
(7),
Cl
(7), NH
3
(8),
SCN
(9),
Br
(9
)>
I
(13),
CN
(13),
S
2
O
3
2
(14).
Ligand scale
Ligands assigned by Pearson as hard ap-
pear in normal type, borderline in bold,
and soft in italics. For Pearson, aliphatic
amines are hard and aromatic amines are
borderline. Since imidazole binds metal
ions through a borderline pyridine-type
nitrogen, imidazole is also classed as bor-
derlineintheligandscale.UseofCH
3
Hg
+
in place of Hg
2
+
yields a virtually identi-
cal ordering with a lesser span of 13 log
units.
The
hardest
ligands
appear
at
the
beginning of the ligand scale, which
spans 17 log units. Cyanide and thio-
cyanate
are
ambivalent
ligands;
vir-
tually
all
metal
ions
bind
to
CN
through the carbon atom. Even upon
binding
the
±rst
CN
;C
u
+
,A
g
+
,
and
Hg
2
+
undergo
a
reduction
to
two-coordination. A reduction to four-
coordination occurs upon binding of the
second CN
to Ni
2
+
and Zn
2
+
.Areduc
-
tion in coordination number strengthens
binding.
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