Chemistry Reference
In-Depth Information
are also the most electronegative. Therefore, arranging donor ions or groups in terms of
increasing electronegativity provides us with a trend from 'soft' to 'hard' ligands:
'SOFT'
−
increasing electronegativity
→
'HARD'
CN
−
S
2
−
RS
−
I
−
Br
−
Cl
−
−
OH
F
−
NH
3
OH
2
The ligands at each 'end' show distinct preferences for different types of metal ions. The
F
−
ligand binds strongly to Ti
4
+
, whereas CN
−
binds strongly to Au
+
, and it is clear these
metal ions differ in terms of both ionic radius and charge. Any individual metal ion will
display preferential binding when presented with a range of different ligands. This means
that metal ions can be graded and assigned 'hard-soft' character like ligands; however, by
convention, we define the least electronegative as 'hardest' and the most electronegative as
'softest'. For the metals, they grow increasingly 'soft' from left to right across the Periodic
Table, and also increase in softness down any column of the table. This definition for metals
allows us to apply a simple 'like prefers like' concept -
'hard' ligands (bases) prefer 'hard'
metals (acids)
,'
soft' ligands (bases) prefer 'soft' metals (acids)
. This principle of hard and
soft acids and bases (HSAB), developed by Pearson, is a simple but surprisingly effective
way of looking at experimentally observable metal-ligand preference. To see the concept in
action, we should look at a few examples of how it allows 'interpretation' of experimental
observations; yet, without a strong theoretical basis, the concept is somehow unsatisfying.
One example involves the long-established reaction of Co
2
+
and Hg
2
+
together in solution
with SCN
−
. The ligand has both a 'soft' donor (S) and a 'hard' donor (N) available; although
like-charged, Hg
2
+
is larger than Co
2
+
and further across and down the Periodic Table,
and thus 'softer'. This reaction results in a crystalline solid [(NCS)
2
Hg(
-
NCS)
2
Hg(SCN)
2
], with each metal in a square plane of thiocyanate donors, with four
S atoms bound to each Hg
2
+
and four N atoms bound to the central Co
2
+
. Only this
thermodynamically stable product, where the softer S bonds to softer Hg
2
+
, and harder N
bonds to harder Co
2
+
, is known. If the 'wrong' end of a ligand like thiocyanate binds initially
to a mismatched metal ion, it will usually undergo a rearrangement reaction to reach the
stable, preferred form. Where SCN
−
is forced initially to bond to the 'hard' Co
3
+
through the
'soft' S atom, it undergoes rearrangement to the form with the 'hard' N atom bound readily.
It is possible to apply the concepts exemplified above to reaction outcomes generally. In all
cases it is a comparative issue; for example, a ligand defined as 'harder' versus one other
donor type (such as OH
2
versus Cl
−
) may be considered 'softer' if compared with another
type of ligand (such as NH
3
versus OH
2
); this aspect will also be true for metal ions.
Definition of hard/soft character is the result of empirical observations and trends in
measured stability of complexes. For example, hard acids (such as Fe
3
+
) tend to bind the
halides in the order of complex strength of F
−
-SCN)
2
Co(
I
−
, and soft acids (such as
Hg
2
+
) in the reverse order of stability. However, as with any model with just two categories,
there will be a 'grey' area in the middle where borderline character is exhibited. This is the
case for both Lewis acids and Lewis bases. Selected examples are collected in Table 3.2
below; a more complete table appears later in Chapter 5.
The '
hard
'-'
soft
' concept is, from a perspective of the metal ion, often recast in terms
of two classes of metal ions as follows:
'
Class A
' Metal Ions ('hard')
- These are small, compact and not very polarizable; this
group includes alkali metal ions, alkaline earth metal ions, and lighter and more highly
charged metal ions such as Ti
4
+
,Fe
3
+
,Co
3
+
,Al
3
+
. They show a preference for ligands
(bases) also small and less polarizable.
Cl
−
Br
−
