Chemistry Reference
In-Depth Information
but it has the capacity to lose protons and thus act as an acid,
H
+
+
−
NH
2
NH
3
(p
K
a
20)
although this reaction has such a high p
K
a
that it is not of significance for free ammonia
in water. However, as for coordinated water, acidity of ammonia is significantly enhanced
through coordination,
M
n
+
−
H
+
+
M
n
+
−
−
NH
2
NH
3
(p
K
a
≈
10
−
14)
so that sufficient concentrations of bound amide can form to permit reaction. Overall, the
coordinated amide anion is a far better nucleophile than free amide ion. Alkylamines can
show the same activity as nucleophiles,
M
n
+
−
H
+
+
M
n
+
−
−
NR
2
NHR
2
as long as they have at least one amine hydrogen atom to release as a proton.
Reactions of coordinated ligands with organic substrates usually occur where the organic
molecule enters the coordination sphere in a position adjacent to the nucleophile, and the
subsequent reaction involves attack of the coordinated nucleophile at a relatively electron-
deficient site on the organic substrate. These reactions lead to a new organic molecule that
is usually chelated to the metal ion. This product may depart from labile complex centres
through substitution by other ligands (providing a mechanism for repeating the reaction,
or catalysis), or else may occur with inert metal centres as a single stoichiometric reaction.
These reactions can also induce a particular stereochemistry, and may be defined as
stere-
ospecific
(producing exclusively one isomer) or
stereoselective
(producing an excess of one
isomer).
Selectivity
can be introduced simply by preference for a particular conformation
in a chelate ring equilibrium, as illustrated in Figure 6.8.
Here the
con-
formation (right hand side, Figure 6.8) of the chelate ring, as steric clashing (of the ring
methyl substituent with other axial ligands on the complex) is minimized in the former. Any
subsequent reaction will 'carry forward' this selectivity into the reaction outcome, leading
to selectivity in the product.
Either a coordinated
−
OH or
−
NH
2
group is able to initiate chemistry with appropriate
ligands present in an adjacent (
cis
) site. This reactivity was probed in detail for several
conformation (left hand side, Figure 6.8) is preferred and not the
potential steric interaction
λ
δ
Figure 6.8
Two conformations of a substituted ethylenediamine chelate ring. The left-hand conformer has the
methyl substituent on the chelate ring displaced away from the rest of the molecule (equatorial),
whereas the right-hand conformer places it in an axial disposition where it may clash with another
axial ligand, which is thus unfavourable.
