Chemistry Reference
In-Depth Information
TABLE 3.9
Rate Constants for Water Exchange in Aqua Complexes
Metal ion
d
n
Hydration Number
a
Water Exchange Rate (s
−1
)
a
Group 1A
Li
+
—
4
∼10
9
Na
+
—
6
∼10
9
K
+
—
6
∼10
9
Group 2A
Mg
2+
—
6
∼10
6
Ca
2+
—
8
∼10
8
Sr
2+
—
6
∼10
9
Ba
2+
—
6
∼10
9
1
st
Row Transition Metals
Cr
2+
d
4
6
∼10
9
Cr
3+
d
3
6
2.4 x 10
−6
Mn
2+
d
5
6
2.1 x 10
7
Fe
2+
d
6
hs 6
4.4 x 10
6
Fe
3+
d
5
hs 6
∼10
−3
Co
2+
d
7
6
3.2 x 10
6
Co
3+
d
6
hs 6
∼10
−1
Ni
2+
d
8
6
3 x 10
4
Cu
2+
d
9
6
4.4 x 10
9
Zn
2+
d
10
6
∼10
7
2
nd
Row Transition Metals
Ru
3+
d
5
6
1.8 x 10
−2
Pd
2+
d
8
4
5.6 x 10
2
Cd
2+
d
10
6
∼10
8
3
rd
Row Transition Metals
Pt
2+
d
8
4
3.9 x 10
−4
Hg
2+
d
10
6
∼10
9
Source:
Adapted from A.L. Feig and O.C. Uhlenbeckv. “The Role of Metal Ions in
RNA Biochemistry, in
The RNA World.
2nd edition, 287-319 (New York:
Cold Spring Harbor Laboratory Press, 1999).
other ligand (Table 3.9) vary over a wide range, from very high rates (k ≈ 10
9
M
−1
s
−1
)
to very low rates (k ≈ 10
−9
M
−1
s
−1
).
The analysis of the data in Table 3.9 shows that:
1. Nontransition metal aqua complexes are extremely labile.
The size of the central ion influences ligand replacement. The small central
ions are held tightly by the ligand, resulting in more inert complexes.
For example, the rate constant for water exchange in the three aqua
