Bioinorganic Chemistry
641
well-defned six-coordinate octahedral
complexes with precise bond lengths
and Ca
2
+
Forms looser complexes oF
higher and more variable coordination
number, without directionality and with
variable bond lengths.
4. Though both ions Favor coordination
at oxygen donor ligands, Mg
2
+
binds
more strongly than Ca
2
+
to small
ligands with nitrogen donors. With re-
spect to the pair oF related tridentate
ligands oxydiacetate and iminodiac-
etate, Ca
2
+
binds more strongly to the
Former and Mg
2
+
to the latter N-donor
ligand. The stability constant For Mg
2
+
binding is greater For iminodiacetate
than For oxydiacetate, while For Ca
2
+
the reverse is true. Mg
2
+
binds to Four
nitrogen donors in chlorophyll, while
Ca
2
+
does not.
5. Ca
2
+
is more comFortable than Mg
2
+
in
binding to large, multidentate, anionic
ligands. Although Mg
2
+
binds more
strongly than Ca
2
+
to iminodiacetate,
Ca
2
+
binds more strongly to nitrilo-
triacetate, EGTA, and EDTA. Ca
2
+
binds to EGTA a striking 10
5
.
6
times
more strongly than does Mg
2
+
.Ina
l
l
three ligands, there are steric require-
ments that the larger Ca
2
+
Fulflls better
than Mg
2
+
.Mg
2
+
binds slightly more
strongly to nucleoside phosphates,, but
within cells, there is insuFfcient Free
Ca
2
+
For binding. Within cells, Mg
2
+
associates with phosphates and Ca
2
+
binds tightly to proteins.
The equilibrium picture shows Mg
2
+
Forming more restrictive complexes,
usually hexacoordinate with precise
bond lengths, Favoring smaller chelate
groupings, and more readily accept-
ing nitrogen donors. In contrast, Ca
2
+
Forms less well-defned complexes with
eight-, six-, and other Fold coordina-
tion with variable bond lengths and
Favors larger multidentate anionic lig-
ands. These equilibrium stability and
structural diFFerences account For diF-
Ferentiation between Ca
2
+
and Mg
2
+
in biological systems.
6. Ca
2
+
undergoes substitution at least
10
3
times
Faster
than
Mg
2
+
.T
h
e
slower rate For Mg
2
+
probably results
From a more limited capability to aid
substitution by reducing or enlarging
its coordination sphere. Thus, From
both static and dynamic viewpoints,
Mg
2
+
Forms the more prescribed and
tighter complexes.
7. Insolubilities oF their common phos-
phate salts provide only a weak diF-
Ferentiation between Mg
2
+
and Ca
2
+
.
Inorganic Mg
2
+
phosphates are up to
10 times more soluble than Ca
2
+
salts oF
similar structure. Both metal ions Form
soluble salts with organic phosphates.
The main diFFerence between Mg
2
+
and
Ca
2
+
is that, owing to its small size,
Mg
2
+
Fails to Form a structure like the
most insoluble oF the Ca
2
+
phosphates,
hydroxyapatite, Ca
5
(PO
4
)
3
OH.
5
Stability Sequences
±rom the many studies on stability con-
stants, one fnds the order oF metal ion
stabilities to be dependent on the lig-
and. Increasing metal ion stabilities Follow
the orders
Glycine:
Ca
,
Mg
¿
Mn
<
±e
,
Cd
,
Pb
<
Co,
Zn
<
Ni
¿
CH
3
Hg
+
,Cu
¿
Hg
1,2-Diaminoethane:
Mg
¿
Mn
¿
±e
<
Pb
,
Cd
,
Co
,
Zn
¿
Ni
<
CH
3
Hg
+
¿
Cu
¿
Hg
The ion Ca
2
+
does not Form stable amine
complexes. Except For methyl mercury,
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