Chemistry Reference
In-Depth Information
an amine hydrogen atom, arising from the interaction
-
O-CO-R
R
2
N-H
+
ยทยทยท
.
Such directed close contacts can actually be observed in the crystal structure also, but
distances are longer than formal covalent bonding distances and the bonds much weaker.
Some ligands are structurally so rigid that they can bind to a metal ion in only one
manner. The aromatic porphyrin molecule is completely flat, and large amounts of energy
are required to distort it. Therefore, when it binds to a metal, it seeks to retain this shape,
and will simply use the square-shaped array of four N-donors to wrap around a metal ion in
a planar manner. Not only aromatic ligands are rigid; some polycyclic fused-ring aliphatic
molecules may be sufficiently rigid and require a particular shape. An example of a cyclic
rigid four nitrogen donor ligand that can bind effectively only with the square-shaped array
of four N-donors in the plane about the metal ion (Figure 4.22) contrasts with the flexible
aliphatic ligand also shown, which can bind in a flat or 'bent' arrangement. Even ligands
where the donors do not form part of a ring are affected by rigidity; the tridentate ligand
at the right in the figure is conjugated and must preferentially remain flat, limiting the way
X
O
X
O
M
N
X
O
-
mer
(ligand flat)
HN
O
-
N
X
O
X
X
-
O
M
-
O
N
O
fac
(ligand folded)
X
N
N
N
M
N
X
NH
HN
trans
(ligand flat)
N
N
N
HN
N
NH
N
N
X
X
M
N
N
cis
(ligand folded)
Figure 4.22
Rigid and flexible ligands influence the way they bind to metal ions. The more rigid ligands on the
right do not permit folding, whereas the more flexible ones on the left can accommodate folding and
thus offer options when coordinating to an octahedral metal ion.
