Chemistry Reference
In-Depth Information
M
M
M
-
O
O
O
O
O
-
O
O
-
O
C
C
C
C
M'
R
R
R
R
free
didendate
chelate
monodentate
didentate
bridging
Figure 2.4
Metal ion binding options for a carboxylate group, featuring various monodentate and didentate
coordination modes.
in the continuous ring, starting and ending at the metal, and including it. For example the
chelated carboxylate in Figure 2.4 involves the sequence M
, with the last
O returning us to the M, so four atoms are involved in the continuous ring, meaning it is a
four-membered chelate ring.
For the diaminomethane of Figure 2.3, like the carboxylate discussed above, the chelate
ring is a four-membered ring, as it involves four atoms (including the metal) linked together
in a ring by four covalent bonds, two of which are coordinate bonds. Just in the way that ring
structures of a certain range of sizes in organic compounds are inherently stable, chelation
leads to enhanced stability in metal complexes for chelate rings of certain sizes.
If, instead of diaminomethane, the much more chemically stable diaminoethane (for-
mally named ethane-1,2-diamine, but also called ethylenediamine or often simply 'en') is
employed, chelation leads to a five-membered chelate ring. For this to happen, first one
nitrogen must form a bond to the metal, then the remaining lone pair must be rotated to
an appropriate orientation and the nitrogen approach the metal so as to lead to effective
binding and hence chelation. The anchoring of the first nitrogen to the metal means the
second one cannot be too far away in any orientation, facilitating its eventual coordination
(Figure 2.5).
Looking along the C C bond of diaminoethane, the two amines must adopt a
cis
dis-
position for chelation; in the
trans
disposition (shown at centre left in Figure 2.6), only
bridging to two separate metals can result. With a flexible ligand like this, rotation about
the C
→
O
→
C
→
O
→
C bond readily permits change from one conformation to another in the free ligand
M
M
H
2
C
H
C
H
H
H
H
H
N
N
N
N
H
H
H
H
H
N
H
2
N
H
2
H
H
H
2
C
2
Figure 2.5
The stepwise process for chelation of diaminoethane. This features initial monodentate formation,
rearrangement and orientation of the second lone pair, and its subsequent binding to form the chelate
ring.
