Chemistry Reference
In-Depth Information
trigonal
bipyramidal
(D
3h
)
square
pyramidal
(C
4v
)
Figure 4.13
The two limiting shapes for five-coordination.
square pyramidal shape where the metal lies above the basal square plane, with typically
the angle from the apical donor through the metal to each donor in the base around 105
◦
instead of 90
◦
. Considering electron pair repulsion alone, this distorted shape (distorted
only in terms of the metal location; the Egyptian pyramid shape created by the donor groups
is otherwise unaltered) is actually more stable than the form created by simply truncating
an octahedron, and is only slightly less stable than the trigonal bipyramidal geometry. As
a consequence, it has become usual to regard the square pyramidal shape as that with the
metal above the pyramidal base plane, and you will see it represented in this way almost
exclusively.
Once again, as described for three- and four-coordination, it is possible to convert
from one form to the other through bond angle changes without any bond-breaking. Both
geometries are common, but in practice there are many structures that are intermediate
between these two. The two limiting structures are of similar energy and as predicted, some
complexes display an equilibrium between the two; [Ni(CN)
5
]
3
−
, for example, crystallizes
with both structural forms of the anion present in the crystals.
If we examine the two five-coordinate shapes from a crystal field perspective, the d
orbitals split in a different way to that found for octahedral, tetrahedral and square planar
shapes since the d orbitals find the ligands in clearly different locations in space. The crystal
field splitting pattern for the two is shown in Figure 4.14. From this pattern, crystal field
stabilization energies can be calculated, and favour the square pyramidal geometry in all
cases (apart from the trivial situations d
0
and d
10
) except for high spin d
5
. This prediction
differs from the outcome from the electron pair repulsion model.
Although electron pair repulsion and crystal field stabilization energy (CFSE) influences
operate, it appears nevertheless that the steric or shape demands of at least polydentate
ligands play a dominant influence on complex shape. This is exemplified in Figure 4.15,
where an example of a 'three-legged' ligand shape fits best to the trigonal bipyramidal
geometry, occupying the top four positions of the complex shape, with a fifth simple ligand
then occupying the underside - the whole assembly looks a little like an open umbrella. The
ligand is pre-disposed to this shape, with limited steric interaction when bound. Likewise,
the cyclic tetraamine represented in Figure 4.15 is predisposed to binding in the square
pyramidal shape.
Overall, trigonal bipyramidal is found rather than square pyramidal shape except where
steric requirements of polydentate ligands are important or where
bonding occurs (as
in vanadyl complexes [VOL
4
] where the V O bond occupies the axial site with four
other ligands in the basal plane). However, exceptions abound, reflecting the similar en-
ergies of the two forms. Examples of complexes with trigonal bipyramidal geometry are
