Chemistry Reference
In-Depth Information
unstable O-bound nitrite rather than the thermodynamically stable N-bound nitrite is sup-
porting evidence. The formed O-bound form does spontaneously isomerize to the N-bound
form in a relatively slow isomerization reaction, but the rate is sufficiently slow that the
O-bound form can be isolated readily.
2+
2+
NH
3
NH
3
O
NH
3
NH
3
16
OH
2
+ H
+
18
OH
+ NO
+
18
O
Co
Co
N
H
3
N
H
3
N
(6.41)
H
3
N
H
3
N
NH
3
NH
3
Other reactions of simple anions that may occur in the absence of coordination can also
be observed to occur for the complexed form, with the rate of reaction usually changed
significantly as a result of complexation. This is anticipated, since a coordinated ion is
bonded directly to a highly-charged metal ion, which must influence the electron distribution
in the bound molecule and hence its reactivity. Two well-known examples where the
product is ammonia occur through either reduction of nitrite with zinc/acid or oxidation of
thiocyanate with peroxide. The former example is exemplified in (6.42) below.
1+
2+
NH
3
NH
3
O
-
Zn / H
+
aq
II
II
(+ 2 H
2
O + Zn
2+
aq
)
Pt
N
Pt
NH
3
(6.42)
H
3
N
H
3
N
O
NH
3
NH
3
Transition metal complexes can promote reactions by organizing and binding substrates.
We have already seen this in terms of metal-directed reactions. Another important function
is the supply of a
coordinated nucleophile
for the reaction, which is incorporated in the
product. We have already seen a coordinated nucleophile at work in the reaction discussed
above of Co
−
OH with NO
+
; nucleophiles, which are electron-rich entities, are best
represented in coordination chemistry by coordinated hydroxide ion, formed by proton
loss from a water molecule; this is a common ligand in metal complexes. Normally, water
dissociates only to a very limited extent, via
H
+
+
−
OH
OH
2
[H
+
][
−
OH]/[H
2
O] and for which p
K
w
≈
for which we define
K
w
=
14.
However, when bound to a highly-charged metal ion, its acidity is very significantly
enhanced, to the extent that, at neutral pH, a coordination complex will have a significant
part of its M
OH
2
present as M
−
OH, via
M
n
+
−
H
+
+
M
n
+
−
−
OH
OH
2
(p
K
a
≈
5
−
9)
Although the coordinated hydroxide is a slightly worse nucleophile than free hydroxide,
due to electronic effects of the bonded metal cation, its substantially higher concentration
in the bound form at any pH more than compensates. A coordinated water molecule with a
p
K
a
of 7 will be 50% in the hydroxide form at neutral pH, for example. Importantly, because
it can often be placed adjacent to a bound substrate (thus
pre-organized
for reaction) it is
very effective, and marked catalysis is commonly observed.
Although the most important, hydroxide is not the sole example of a coordinated nucle-
ophile met in coordination chemistry. The next most important, as a result of the prevalence
of ammonia as a ligand, is the amide ion. Ammonia is usually thought of simply as a base,
