Chemistry Reference
In-Depth Information
Table 3.2
Selected examples of hard/soft Lewis acids and bases.
Character
Lewis acids
Lewis bases
H
+
,Li
+
,Na
+
,Mg
2
+
,Cr
3
+
,Ti
4
+
F
−
,HO
−
,H
2
O, H
3
N, CO
3
2
−
,PO
4
3
−
Hard
Fe
2
+
,Co
2
+
,Ni
2
+
,Cu
2
+
,Zn
2
+
Br
−
,NO
2
−
,SC
N
−
Intermediate
Soft
Cu
+
,Ag
+
,Au
+
,Hg
+
,Cd
2
+
,Pt
2
+
I
−
,CN
−
,CO,H
−
,
S
CN
−
,R
3
P, R
2
S
'
Class B
' Metal Ions ('soft')
- These are larger and more polarizable; this group includes
heavier transition metal ion such as Hg
2
+
,Pt
2
+
,Ag
+
, as well as low-valent metal ions
including formally M(0) centres in organometallic compounds. They exhibit a preference
for larger, polarizable ligands. This leads to a preference pattern outlined in Figure 3.27.
There is also a changing preference order seen across the rows of the Periodic Table, with
Class A
showing a trend from weaker to stronger from left to right, and
Class B
showing
the opposite trend of stronger to weaker, defined in terms of the measured stability constant
of ML complexes formed.
3.5.3
Complex Lifetimes - Together, Forever?
Metal complexes are not usually unchanging, everlasting entities. It is true that many, once
formed, do not react readily. However, the general observation is that coordinate bonds
are able to be broken with more ease than cleaving a covalent carbon-carbon bond. The
strength of metal-donor bonds are typically less than most bonds in organic molecules, but
much greater than the next most stable type, hydrogen bonds. We recognize carbon-carbon
bonds as strong and usually unchanging and hydrogen bonds as weak and easily broken; the
coordinate covalent bond lies in the middle ground, albeit nearer the carbon-carbon end of
the park. Breaking a metal-ligand bond is almost invariably tied to a follow-up process of
making another metal-ligand bond in its place, so as to preserve the coordination number
and shape of the complex. When the same type of ligand is involved in each sequential
process,
ligand exchange
is said to have occurred. Where the ligands and solvent are one
and the same, there is no opportunity for any alternative reaction. Even at equilibrium, metal
complexes in solution display a continuous process of ligand exchange, where the rate of
the exchange process is driven by the type of metal ion, ligands and/or solvent involved.
Complexes
of
Clas
s
A
Metal Ions
Complexes
of
Class B
Metal Ions
Ligands
R
3
N
R
2
O
F
-
strongest
weakest
R
3
P
R
2
S
Cl
-
R
3
As
R
2
Se
Br
-
R
3
Sb
R
2
Te
I
-
weakest
strongest
Figure 3.27
Metal-ligand preferences for key ligands. In any column, a
Class A
metal ion prefers ligands from
the top whereas a
Class B
metal ion prefers ligands from the bottom.
