Chemistry Reference
In-Depth Information
bind to
soft bases to give FMO-controlled (covalent) complexes. These interac-
tions are dominated by the energies of the participating FMOs, the HOMO, and
the LUMO.
In other words, hard acid-hard base interactions are predominantly electrostatic
and soft acid-soft base interactions are predominantly covalent.
The main contribution of Klopman theory is that the contributing aspects of
charge-controlled and FMO-controlled Lewis acid-base complexation are separated
and quantified.
Based on the research of Klopman, and Parr and Pearson (Klopman 1968; Parr
and Pearson 1983), Pearson described the absolute hardness (η) quantitatively as
being proportional to the difference between I (ionization potential) and A (electron
affinity) of the species (Pearson 1988). Absolute softness is defined as η
-1
. The abso-
lute electronegativity (χ) and the absolute hardness (η) are applied quantitatively to
any given acid-base reaction. Table 3.10 presents χ and η values for some represen-
tative metal ions.
A recent contribution to the HSAB principle is the application of concepts of local
softness and local hardness (Torrent-Sucarrat et al. 2010). Instead of global hard-
ness (η) and global softness (S) (see Equations [3.15] and [3.16]), which are called
global descriptors
because they describe the properties of a molecule as a whole, the
local descriptors are used.
TABLE 3.10
Absolute Hardness and Absolute Electronegativity for Some Metal Ions
Absolute
Hardness
η
Absolute
Electronegativity
χ
Absolute
Hardness
η
Absolute
Electronegativity
χ
Metal
Metal
Group 1A
Group 2A
Li
+
35.12
40.52
Mg
2+
47.59
32.55
Na
+
26.21
21.08
Ca
2+
19.52
31.39
K
+
17.99
13.64
Sr
2+
27.3
16.3
1
st
Row Transition Metals
2
nd
Row Transition Metals
Cr
2+
7.23
23.73
Ru
3+
10.7
39.2
Cr
3+
9.1
40.0
Pd
2+
6.75
26.18
Mn
2+
9.02
24.66
Cd
2+
10.29
27.20
Fe
2+
7.24
23.42
Fe
3+
12.08
42.73
3
rd
Row Transition Metals
Co
2+
8.22
25.28
Pt
2+
8.0
27.2
Co
3+
8.9
42.4
Hg
2+
7.7
26.5
Ni
2+
8.50
26.67
Miscellaneous
Cu
+
6.28
14.01
Pb
2+
8.46
23.49
Cu
2+
8.27
28.56
Tl
+
7.16
13.27
Zn
2+
10.88
28.84
Source:
Data from R.G. Pearson, “Absolute Electronegativity and Hardness: Application to
Inorganic Chemistry.
Inorg. Chem.
27, no. 4 (1988):734-740.
