Bioinorganic Chemistry
647
The above order of increasing softness
o
fth
eam
in
e
si
sa
l
s
oth
eo
r
d
e
ro
fin
-
creasing basicity. The influence of greater
basicity in yielding increasing softness is
also illustrated by two other comparisons
in the ligand scale: H
2
O(0
)<
OH
−
(5)
and HCO
3
−
(4
)<
CO
3
2
−
(5). These
se-
quences
point
a
general
feature
of
metal
ion
stabilities:
greater
basicity
yields greater metal ion stability, which
in
turn
provides
greater
discrimina-
tion among metal ions, which in turn
gives rise to softness. Often, however,
other factors overwhelm the influence
of
basicity:
among
the
halides,
fluo-
ride is both the strongest base and the
hardest.
In the ligand scale, the relative soft-
ness of the presumably hard ammonia(8),
comparable to borderline bromide(9) and
soft thiocyanate(9), emerged unexpectedly.
The above ligand scale serves as a quanti-
tative indication of the relative hardness
of ligands. The 2 inequality signs di-
vide the 16 members of the ligand scale
into 3 groups on the basis of the mag-
nitude of the differences. The Frst 6
ligands are assigned as hard by Pearson;
of the next closely spaced 7 ligands, 2
are hard, 4 borderline, and 1 soft. The
ligand scale ends with 3 soft ligands.
On the basis of the quantitative results
of the ligand scale, previously hard am-
monia and chloride and soft thiocyanate
a
remoreapp
rop
r
ia
te
lysw
i
tchedin
tothe
borderline group. These switches more
consistently place all amines, aliphatic
and aromatic, in the same borderline
group. The recommended designations
lead
to
a
consistent
sequence
of
6
hard, 7 borderline, and 3 soft ligands.
Thus, all oxygen donor ligands are des-
i
g
n
a
t
e
dh
a
r
da
n
da
l
ln
i
t
r
o
g
e
nd
o
n
o
r
s
borderline. Within the borderline group,
aliphatic amines are softer than aromatic
amines.
8.5
Lead
Lead (Pb
2
+
) presents a case that demon-
strates limitations in the hard–soft con-
cept.
The
Frst
stability-constant
loga-
rithms for lead and the halide ions, all
at 25
◦
C and 1.0 M ionic strength, ap-
pear in parentheses: ±(1.44)
>
Cl(0.90)
<
Br(1.10)
<
I(1.26). The increasing trend
for the last three halides describes class
(b) behavior, while the greatest value for
±
−
characterizes a class (a) metal ion.
Pb
2
+
interacts relatively strongly with both
oxygen (hard) and sulfur (soft) donor lig-
ands, and relatively weakly with nitrogen
donor ligands, as illustrated in the stabil-
ity ruler. Despite the presence of a free
sulfhydryl group on the proteins, Pb
2
+
binding occurs exclusively at the Ca
2
+
sites (composed solely of O donors) of on-
comodulin and chick vitamin D–induced
intestinal calcium-binding protein. Pb
2
+
also combines with the components of
nucleic acids. In many examples, Pb
2
+
es-
chews borderline (is antiborderline) and
opts for either hard or soft behavior,
making the simple hard–soft concept in-
effective for use with this metal ion. In
contrast, compared to other metal ions,
Ni
2
+
exhibits a tendency to prefer nitro-
gen over oxygen or sulfur donors (except
when two or more sulfur donors promote a
diamagnetic complex of reduced coordina-
tion number). The position of the proton
swings from one end to another of the
several metal ion scales. Much more than
Pb
2
+
, the proton displays strong antibor-
derline behavior appearing at either the
hard or the soft ends of the scales. Thus,
biology offers metal ion environments in-
consistent with and unpredictable under
