Geoscience Reference
In-Depth Information
Table 6.1 Hard and soft Lewis acids and bases in solution (Sposito
1981
)
Lewis acids
Hard acids
H
+
,Li
+
,Na
+
,K
+
(Rb
+
,Cs
+
), Mg
2+
,Ca
2+
,Sr
2+
(Ba
2+
), Ti
4+
,Zr
4+
,Cr
3+
,Cr
6+
, MoO
3+
,Mn
2+
,
Mn
3+
,Fe
3+
,Co
3+
,Al
3+
,Si
4+
,CO
2
Borderline acids
Fe
2+
,Co
2+
,Ni
2+
,Cu
2+
,Zn
2+
(Pb
2+
)
Borderline acids
Fe
2+
,Co
2+
,Ni
2+
,Cu
2+
,Zn
2+
(Pb
2+
)
Soft acids
Cu
+
,Ag
+
,Au
+
,Cd
2+
,Hg
+
,Hg
2+
,Ch
3
Hg
+
; pi-acceptors such as quinones; bulk metals
Lewis bases
Hard bases
NH
3
, RNH
2
,H
2
O, OH
-
,O
2-
, ROH, CH
3
COO
-
,CO
3
2-
,NO
3
-
,PO
4
3-
,SO
4
2-
,F
-
Borderline bases
C
6
H
5
NH
2
,C
2
H
5
N, N
2
,NO
2
-
,SO
3
2-
,Br
-
(Cl
-
)
Soft bases
C
2
H
4
,C
6
H
6
,R
3
P (RO)
3
P, R
3
As, R
2
S, RSH, S
2
O
3
-
,S
2-
,I
-
R = organic molecular unit. ( ) indicates a tendency to softness
The principle of hard and soft Lewis acids and bases, proposed by Pearson
(
1963
), is useful to describe these reactions. A Lewis acid is any chemical species
that employs an empty electronic orbital available for reaction, while a Lewis base
is any chemical species that employs a doubly occupied electronic orbital in a
reaction. Lewis acids and bases can be neutral molecules, simple or complex ions,
or neutral or charged macromolecules. The proton and all metal cations of interest
in subsurface aqueous solutions are Lewis acids. Lewis bases include H
2
,O
2
,
oxyanions, and organic N, S, and P electron donors. A list of selected hard and soft
Lewis acids and bases found in soil solutions is presented in Table
6.1
.
An inner-sphere complex is formed between Lewis acids and bases, while an
outer-sphere complex involves a water molecule interposed between the acid and
the base. A hard Lewis acid is a molecular unit of small size, high oxidation state,
high electronegativity, and low polarizability; whereas a soft Lewis acid is a
molecular unit of relatively large size, characterized by low oxidation state, low
electronegativity, and high polarizability. Based on this characterization, hard
bases prefer to complex hard acids, and soft bases prefer to complex soft acids,
under similar conditions of acid-base strength.
The presence of outside ligands in the subsurface aqueous solution leads to an
increase in the solubility of coordinating ions. A complex with any ligand, L
n
,or
its protonated form, H
k
L
n
, has a total solubility expressed by
A ½A
free
þ
X
i
½A
m
H
k
L
n
ð
OH
Þ
i
;
ð
6
:
6
Þ
where A represents the ion to be coordinated with different ligand types L
n
and all
values of m, n,ork C 0 must be considered in the summation.
