Chemistry Reference
In-Depth Information
Figure 4.1
A view of the structure determined by X-ray crystallography of the simple neutral complex
[PdBr
2
(thiomorpholine)
2
]. The atom locations are represented by probability surfaces, with the
smaller the size the better defined is the atom. (Reprinted with permission from Australian Jour-
nal of Chemistry, 'Complexation of constrained ligands piperazine, N-substituted piperazines and
thiomorpholine' by Sarah E. Clifford, Geoffrey A. Lawrance, et al., 62, 12 Copyright © (2009)
CSIRO Publishing)
described in terms of a different basic shape. There are obviously several factors influencing
the outcomes we see experimentally.
There are some basic ideas we can immediately introduce to explain the experimental
observations. Consider the simple cation [Co(NH
3
)
6
]
3
+
.AsanML
6
compound, we would
predict initially (see Figure 3.3) that this complex cation will be octahedral in shape. From X-
ray crystallography, this is exactly what is found; all N Co N angles between neighbouring
ammonia groups are (or at least are very close to) 90
◦
, and in additional all M N bond
distances are identical within very small margins of error. Experiment has justified use of
our simple point-charge model. Now consider what happens if just one ammonia ligand is
replaced by a bromide ion, to form [CoBr(NH
3
)
5
]
2
+
. The basic shape is still octahedral,
but there are some changes found. First, the Co N distances are different from the Co Br
distance. This is reasonable, since we are, after all, linking different species of different
sizes, charges and shapes. However, the intraligand angles also change, with N-Co-Br
angles opening out a little to be greater than 90
◦
and some N Co N angles closing up
a little to be less than 90
◦
. This implies that the interactions 'sideways' between types of
ligands differ - an ammonia and a halide interact in a nonbonding manner (or push against
each other) differently than do two neighbouring ammonia molecules. We can define these
effects between neighbouring ligands as
ligand-ligand repulsion forces
, called generally a
steric effect
. Simply, two ligands cannot occupy the same space, and compromises must be
reached which involve bond angle distortion and bond length variation. In addition, there
are other effects that are electronic in character. It is apparent that the Co N distance for
the ammonia bound directly opposite the bromide ion (
trans
) differs from those of the four
bound adjacent to the bromide ion (
cis
). This could arguably be related to steric effects
for
cis
groups differing from
trans
groups, but may also be related to electronic effects;
in the simplest view, consider the latter as reflecting the way different ligands compete
differently for d orbitals, or 'push' or 'pull' electron density to or from the metal centre.
