Chemistry Reference
In-Depth Information
TABLE 3.7
Coordination Numbers (CN), Geometries, and Preferred Ligands for
Selected Metal Ions in Biological Systems
Metal Ion
Coordination Number
Geometry
Ligand Atom Donor
Na
+
6
Octahedral
O
K
+
6-8
Flexible
O
Mg
2+
6
Octahedral
O
Ca
2+
6-8
Flexible
O
Mn
2+
(d
5
)
6
Octahedral
O, N
Mn
3+
(d
4
)
6
Tetragonal
O
Fe
2+
(d
6
)
4
Tetrahedral
O
6
Octahedral
O, N
Fe
3+
(d
5
)
4
Tetrahedral
S
6
Octahedral
O
Co
+
(d
8
)
6
Octahedral
O, N
(usually missing
the 6th ligand)
Co
2+
(d
7
)
4
Tetrahedral
S, N
6
Octahedral
O, N
Co
3+
(d
6
)
6
Octahedral
O, N
Ni
2+
(d
8
)
4
Square planar
S, N
6
Octahedral
Uncommon
Cu
+
(d
10
)
4
Tetrahedral
S, N
Cu
2+
(d
9
)
4
Tetrahedral
S, N
4
Square planar
O, N
5
Square pyramidal
O
6
Tetragonal
N
Zn
2+
(d
10
)
4
Tetrahedral
O, S, N
5
Square pyramidal
O, N
Mo
4+
(d
2
)
6
Octahedral
O
Mo
5+
(d
1
)
6
Octahedral
O
Mo
6+
(d
0
)
6
Octahedral
S
Source:
Adapted from J.A. Cowan,
Inorganic Biochemistry: An Introduction.
(New York: Wiley-VCH
Inc., 1996) and R.M. Roat-Malone,
Bioinorganic Chemistry: A Short Course
. Hoboken, NJ:
Wiley, 2003).
3.3.4 t
HErmodynamic
s
tability
of
m
Etal
c
omplExEs
The thermodynamic stability of a metal complex may be represented by a stepwise
formation constant (or stability constant) K
n
or an overall stability constant β
n
. The
formation of a complex between a metal ion, M, and a ligand, L, is usually a substi-
tution reaction. For example, metal ions in aqueous solution will be present as aqua
ions. The reaction for the formation of the first complex could be written as:
[M(H
2
O)
n
] + L ⇌ [M(H
2
O)
n−1
L] +H
2
O
(3.38)
