Chemistry Reference
In-Depth Information
TABLE 3.5
Pearson Classification of Lewis Acids
Represented by Metal Ions
Hard (Class [a])
Soft (Class [b])
H
+
, Li
+
, Na
+
, K
+
Cu
+
, Ag
+
, Au
+
, Tl
+
, Hg
+
, Cs
+
Be
2+
, Mg
2+
, Ca
2+
, Sr
2+
, Mn
2+
Pd
2+
, Cd
2+
, Pt
2+
, Hg
2+
Al
3+
, Sc
3+
, Ga
3+
, In
3+
, La
3+
Tl
3+
, Au
3+
, Te
4+
, Pt
4+
Cr
3+
, Co
3+
, Fe
3+
, As
3+
, Ce
3+
Si
4+
, Ti
4+
, Zr
4+
, Th
4+
, Pu
4+
Ce
4+
, Ge
4+
, VO
2+
UO
2
2+
Borderline
Fe
2+
, Co
2+
, Ni
2+
, Cu
2+
, Zn
2+
, Pb
2+
, Sn
2+
Sb
3+
, Bi
3+
, Rh
3+
, Ir
3+
, Ru
2+
, Os
2+
TABLE 3.6
Pearson Classification of Lewis Bases
Hard Soft
H
2
O, OH
−
, F
−
R
2
S, RSH, RS
−
CH
3
CO
2
−
, PO
4
3−
, SO
4
2−
I
−
, SCN
−
, S
2
O
3
2−
Cl
−
, CO
3
2−
, ClO
4
−
, NO
3
−
R
3
P, R
3
As, (RO)
3
P
ROH, RO
−
, R
2
O CN
−
, RNC, CO
NH
3
, RNH
2
, N
2
H
4
C
2
H
4
, C
6
H
6
H
−
, R
−
Borderline
C
6
H
5
NH
2
, C
5
H
5
N, N
3
−
, Br
−
, NO
2
−
, SO
3
2−
, N
2
In a similar manner, the bases are classified as hard, borderline, or soft
(Table 3.6).
A hard base has a small, electronegative atomic center (χ = 3.0 − 4.0) and is a weakly
polarizable species that is difficult to oxidize. A soft base has a large atomic center
of intermediate electronegativity (χ = 2.5 − 3.0) and it is easy to polarize and oxidize.
Between the two main categories of acids and bases are species with intermediate
character.
Besides the classification of acids and bases, the Pearson principle introduced the
idea of binding preference, namely: hard acids prefer to bind to hard bases, and soft
acids prefer to bind to soft bases.
The knowledge of coordinative binding preference of metal ions is important rela-
tive to the capacity of metal ions to form stable and irreversible coordination com-
plexes with various donor atoms of the macromolecules, and therefore notionally,
metal toxicity. In biological systems, hard metal cations prefer oxygen donor atoms
that are present in groups such as −OH, −COOH, −PO
4
3−
or nitrogen donor atoms
