Chemistry Reference
In-Depth Information
square planar
tetrahedral
A
A
A
A
A
B
B
B
A
B
D
D
A
A
M
M
M
M
M
M
C
C
B
B
A
A
B
B
B
B
cis
trans
geometric isomers (diastereomers)
optical isomers (enantiomers)
(non-superimposable mirror images)
Figure 4.29
Isomers possible with four-coordination. The square planar geometry, with a plane of symmetry,
cannot exhibit optical isomerism but can display geometric isomerism, whereas tetrahedral geometry,
with its symmetrical disposition of bonds, cannot exhibit geometric isomerism but may display optical
isomerism.
complexes can have only enantiomers; they cannot, as a result of their shape, have
cis
and
trans
isomers. Chirality will occur for complexes with four nonequivalent ligands (although
there appear to be no known examples isolated yet)
or
with two unsymmetrical didentate
ligands (Figure 4.29).
4.4.3.2
Six-Coordinate Octahedral Complexes
Six-coordination presents greater options for location of ligands around the coordination
sphere than occurs in four-coordination, as a result of the greater number of donor sites.
Whereas there is only one form of MA
6
and of MA
5
B possible, for MA
4
B
2
there arise
two geometric isomers,
cis
and
trans
(Figure 4.30), although there are no enantiomers as
both diastereomers have planes of symmetry. When we extend to MA
3
B
3
, there are two
geometric isomers,
facial
(abbreviated as
fac
) and
meridional
(abbreviated as
mer
), neither
of which has an enantiomer. If we add additional ligand types, however, the number of
isomers can increase rapidly. For MA
2
B
2
C
2
there are now five diastereomers, one of which
has an enantiomer (Figure 4.30). With more diverse sets of ligands, the number of possible
isomers can grow even greater.
Simple examples of MA
4
B
2
and MA
3
B
3
complexes are those of cobalt(III) formed with
ammonia and chloride ion as ligands, namely the cations
cis
- and
trans
-[CoCl
2
(NH
3
)
4
]
+
and the neutral molecules
fac
- and
mer
-[CoCl
3
(NH
3
)
3
]. These are illustrated with ball-
and-stick drawings, based on X-ray crystal structures of the complexes, in Figure 4.31.
What should be clear from these drawings is that the
fac
isomer carries three of each type
of ligand in an equilateral triangular arrangement on a separate face of the octahedron;
hence the origin of the name. Conversely, the
mer
isomer has each of one type of ligand
lying in a plane that includes the metal, at right-angles to the plane of the other type of
ligand.
The introduction of chelate ligands usually acts to limit the number of geometric isomers,
or at least to not extend the number. For example, the coordination of three symmetrical
didentate chelates can occur in only one way, in the same sense that six identical monoden-
tate ligands can coordinate in only one way. Likewise, M(AA)
2
B
2
compounds (where (AA)
refers to a symmetric chelate) has precisely the same number and type of diastereomers as
