Chemistry Reference
In-Depth Information
Figure 4.32
Non-superimposable mirror image forms (enantiomers) of the diastereomer
cis
-[CoCl
2
(en)
2
]
+
.The
enantiomers are named
and
respectively.
trans
diastereomer) has a mirror image that is non-superimposable on the original, a key
requirement for optical activity (Figure 4.32).
So far, we have introduced the concept of optical activity (or chirality) in a complex
arising as a result of the way ligands are arranged around the central metal. For six-
coordination octahedral, there are actually several ways in which dissymmetry (chirality)
can arise, namely from:
1. the distribution of monodentate ligands about the metal (which occurs with at least three
pairs of different ligands, as exemplified for MA
2
B
2
C
2
in Figure 4.30);
2. the distribution of chelate rings about the metal (as occurs when at least two didentate
chelates are coordinated, as exemplified in Figure 4.32);
3. coordination of unsymmetrical polydentate ligands (where the assembly of different
donors and linkages resulting means the complex becomes dissymmetric);
4. conformations of chelate rings (where tetrahedral carbon and other atoms enforce their
geometry in the chain linking donor groups, causing chelate rings to be nonplanar and
able to adopt conformations that have
mirror images forms - see Figure 2.7);
5. coordination of an asymmetric and hence chiral organic ligand (whereby the chirality
of the part is assigned to the whole) - this chirality in the ligand may be conventional
(arising from asymmetric carbon centres of D or L form), or arise from binding to a
helical ligand (which has
P
and
M
isomers associated with opposite helicities);
6. coordination of a donor atom that is asymmetric (which leads to induced chirality in
the whole complex, similar to the situation in (5) above, but with the asymmetric centre
actually attached to the metal in this case).
and
Examples of several of these cases appear in Figure 4.33, but detailed discussion will
not be pursued here, this being a task for more advanced texts.
4.4.4
What's Best? - Isomer Preferences
Polydentate ligands present particular problems, as a range of geometric isomers, often with
enantiomers, may exist in principle. This is illustrated in Figure 4.34 for a simple system
where a symmetrical tetradentate is bound. This can occur in three ways, two of which
