640
Bioinorganic Chemistry
Tab. 3
Effective ionic radii in picometers.
Ion
Coordination number
a
456789
Be
2
+
27
45
Al
3
+
39
48
54
Cr
3
+
62
Ga
3
+
47
55
62
Fe
3
+
49
58
64
b
78
Ni
2
+
55
63
69
Mg
2
+
57
66
72
89
Cu
2
+
57
65
73
Co
2
+
58
67
74
90
Zn
2
+
60
68
74
90
Li
+
59
76
92
Fe
2
+
63
78
c
92
Mn
2
+
66
75
83
90
96
Lu
3
+
86
98
103
Gd
3
+
94
100
105
111
Cd
2
+
78
87
95
103
110
Ca
2
+
100
106
112
118
La
3
+
103
110
116
122
Na
+
99
100
102
112
118
124
Sr
2
+
118
121
126
131
Pb
2
+
98
119
123
129
135
Ba
2
+
135
138
142
147
K
+
137
138
146
151
155
Tl
+
150
159
Rb
+
152
156
161
163
Cs
+
167
174
178
a
Radius of the most common coordination number is underscored.
b
High spin; low-spin value is 55.
c
High spin; low-spin value is 61.
1. Ca
2
+
favors a higher-fold coordination
number than Mg
2
+
. The favored coor-
dination number for Mg
2
+
is sixfold
octahedral. Ca
2
+
exhibits a greater di-
versity of coordination numbers, with
eight- and sixfold coordination being
the most common with small ligands.
Since coordination in both metal ions
is mainly ionic, the greater coordina-
tion numbers for Ca
2
+
derive ultimately
from its larger size.
2. Ca
2
+
is substantially larger than Mg
2
+
.
In the same coordination number, the
effective ionic radii of Ca
2
+
are at
least 23 pm greater than those of
Mg
2
+
(Table 1.) If the most favored
coordination numbers are considered,
Ca
2
+
(eightfold) possesses a 40 pm
greater effective ionic radius than Mg
2
+
(sixfold).
3. Ca
2
+
to
ligand
donor
atom
bond
distances vary more than those of
Mg
2
+
. Bond distances to oxygen donor
atoms typically range from only 20 to 21
pm for Mg
2
+
and a broader 23 to 26 pm
for Ca
2
+
.Thus
,Mg
2
+
prefers to form
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