Chemistry Reference
In-Depth Information
AgX, which follows the order (log values of the formation constant in parentheses):
F
−
(0
Cl
−
(3
Br
−
(4
I
−
(8
.
3)
.
3)
.
5)
.
0)
.
Clearly, this does not follow the trend expected on electrostatic grounds, which should be
opposite to that observed. The trend is thought to reflect increased covalent character in the
Ag X bond in moving from fluoride to iodide.
The steric bulk of ligands also introduces an influence that can act counter to a pure
base strength effect. This has been discussed earlier for NR
3
compounds (Chapter 2.2.2).
Substituted pyridines present another example of this influence; 2,6-dimethylpyridine is a
poor ligand due to the location of the two methyl groups either side of the pyridine N-donor
atom, despite a similar base strength to unsubstituted pyridine.
5.1.2.2.2 Chelate Effect
From a purely thermodynamic viewpoint, the equilibrium constant is reporting the heat
released (the enthalpy change,
H
0
) in the reaction and the amount of disorder (called
S
0
) resulting from the reaction, related to the reaction free energy as
defined in Equation (5.7). The greater the amount of energy evolved in a reaction the more
stable are the reaction products; this heat change can sometimes be felt when holding a
test-tube in which a reaction has been initiated, and is certainly experimentally measurable
to high levels of accuracy. Further, the greater the amount of disorder resulting from a
reaction the greater is the entropy change and the greater the stability of the products. This
is a harder concept to grasp than some, but think of it in terms of particles involved in a
reaction - if there are more particles present at the end of the reaction than at the start,
or even if those present at the end are less structured or restricted in their locations, there
is increased disorder and hence a positive entropy change. Again, this is experimentally
measurable, but not as directly as a simple heat change associated with reaction enthalpy.
In the coming together of two oppositely charged ions to form a complex, there is both
a release of heat (increased enthalpy,
the entropy change,
H
0
) and a release of solvent molecules from the
ordered and compressed layers around the ions (increased entropy,
S
0
) on complexation.
Moreover, the higher the charge on the metal ion, the greater is the effect.
When we employ molecules as ligands where they offer more than one donor group
capable of binding to metal ions, there is the strong possibility that more than one donor
group will coordinate to the one metal ion. We have entered the realm of polydentate ligands.
Just because a ligand offers two donor groups does not mean that both can coordinate to
the one metal ion. For example, the
para
and
ortho
diaminobenzene isomers can both act
as didentate ligands, but the former must attach to two different metal ions because of the
direction in which the rigidly attached donors point. Only the
ortho
form has the two donors
directed in such a way that they can occupy two adjacent coordination sites around a metal
ion. This isomer has achieved
chelation
. The same behaviour is displayed by the linked
pyridine molecules 2,2
-bipyridine and 4,4
-bipyridine (Figure 5.3).
In general, chelation is beneficial for complex stability, and chelating ligands form
stronger complexes than comparable monodentate ligand sets. For example, consider Equa-
tions (5.9) and (5.10):
Ni(
O
H
2
)
6
2+
+ 6 NH
3
Ni(
N
H
3
)
6
2+
+ 6 H
2
O
[Ni(NH
3
)
6
2+
]
[Ni(OH
2
)
6
2+
] [NH
3
]
6
(5.9)
(
log
β
6
=
8.6)
β
6
=
