Chemistry Reference
In-Depth Information
C
A
A
A
A
A
A
B
A
B
A
B
A
C
A
B
A
B
M
M
M
M
M
M
A
A
A
C
A
A
A
A
A
A
A
A
B
B
B
B
B
C
Figure 4.35
Left
: One of the geometric isomers of a M(AAAA)B
2
complex is transformed into two options when a
mixed donor ligand replaces the common donor ligand in forming a M(AAAC)B
2
complex. Consider
the two arising from replacement of two different terminal donors, so the product has the introduced C
group either opposite a B ligand (left) or opposite an A ligand (right). In reality, of course, the mixed
donor ligand is preformed and coordinates as a single entity.
Right
: One of the geometric isomers
of a M(AAAA)B
2
complex is also transformed into two options when two different monodentate
ligands replace the common monodentate ligands in forming a M(AAAA)BC complex. Consider the
two arising from replacement of two different monodentate donors, so the product has the introduced
C monodentate either opposite a central A group of the tetradentate (left) or opposite a terminal A
group (right).
may occur experimentally. The isomers usually display different total strain energy, and it
is where there are large energy differences that there is observed one thermodynamically
stable form or at most a limited number of forms. Thus only some, and not all, will be
found experimentally, as some may be too strained relative to others to exist.
If we introduce mixed donors into the simple tetradentate AAAA of Figure 4.34, for
example, to form AAAC, the number of isomers will increase. For example, now there
are two different forms of the isomer shown at the left in Figure 4.34, as illustrated in
Figure 4.35. The two new isomers have the same spatial arrangement of the chelate chains,
but the unique terminal groups (A and C) are located differently. An alternative reaction
that increases geometric isomers is where the two common monodentate donor groups
are replaced by two different monodentate donor groups (Figure 4.35, right); there are
again different outcomes depending on which of two groups is replaced. There is adequate
nomenclature to deal with naming these difference diastereomers, but this is not required
to comprehend the outcomes when pictorial representations are available; some aspects of
naming complexes appears in Appendix A. These two simple examples suffice to illustrate
the array of geometric isomers that can result when polydentate and mixed donor ligands
bind to a central metal ion.
Geometric isomers are energetically inequivalent because of a suite of effects that con-
tribute additively to strain in each complex. Sources of strain in complexes are: bond
length deformation (enforced contraction or extension); valence bond angle deformation
(enforced opening/closing up); torsion angle deformations (for chelate rings); out-of-plane
deformations (for unsaturated groups); nonbonded interactions; electrostatic interactions
(of charged groups); and hydrogen bonding interactions. Models that allow estimation
of the strain energies in isomers exist, providing prediction of and/or interpretation of
experimental behaviour. This is addressed later in Chapter 8.3.
4.5
Sophisticated Shapes
The shapes we have described have employed, in all but the last section, simple ligands that
bind at one or two sites around a metal ion. However, most ligands are more complicated
