Chemistry Reference
In-Depth Information
H
2
O
OH
2
OH
2
H
2
O
OH
2
OH
2
M
M
H
2
O
H
H
2
O
H
2
O
OH
2
H
+
M
H
H
N
N
H
- H
2
O
OH
2
H
H
H
N
N
C
N
C
H
H
H
H
C
C
H
H
C
C
H
H
N
H
H
H
H
monodentate
intermediate
H
H
H
H
- 2 H
2
O
H
2
O
OH
2
H
M
H
2
O
OH
2
H
N
M
H
H
H
H
OH
2
N
N
C
- H
2
O
H
C
C
H
H
H
C
N
H
H
H
H
H
H
didentate chelate
Figure 2.18
The process of chelation. A single concerted step (solid arrow) from reactants to products is
not
favoured, but rather a stepwise process (hollow arrows) is involved.
represented by the pathway of hollow arrows in Figure 2.18 is a plausible
mechanism
for the
reaction. Along the way, an
intermediate
compound presumed to form is the monodentate
ethane-1,2-diamine complex (albeit short-lived, because it reacts on rapidly). This is a
viable entity, as this mode of coordination has been seen in some isolable complexes where
only one accessible binding site exists around a metal ion due to other ligands already being
firmly and essentially irreversibly bound in other sites.
The step-by-step process of chelation described above is believed to be the usual way all
polydentate ligands go about coordinating to metal ions in general - 'knitting' themselves
onto the metal ion 'stitch by stitch' (or, more correctly, undergoing
stepwise coordination
).
It also provides a way whereby compounds can achieve only partial coordination of a
potential donor set, when there are insufficient coordination sites for the full suite of
potential donor groups available on a ligand.
Although most of the chelate ligands you will encounter have a carbon backbone and
heteroatoms like nitrogen, oxygen or sulfur as donors, this is not essential. This can best
be illustrated by comparing the two pairs of chelated systems in Figure 2.19, which have
identical ring sizes and shapes, yet one of each pair contains no carbon atoms at all.
S
-
O
-
O
-
S
-
S
H
2
C
M
M
O
C
N
+
M
O
M
S
H
2
C
S
-
O
-
O
-
S
-
Figure 2.19
Simple chelate rings with common donor atoms but different chain atoms.
