Shape - Introduction to Coordination Chemistry - page 86

There are in fact several effects that contribute to the outcome of metal-ligand assembly.
Overall, stereochemistry and coordination number in complexes appear to depend on four
key factors:

1. central metal-ligand electronic interactions, particularly influenced by the number of d
(or f) electrons of the metal ion;
2. metal ion size and preferred metal-ligand donor bond lengths;
3. ligand-ligand repulsion forces;
4. inherent ligand geometry and rigidity.

The simple amended VSEPR point-charge model discussed in Chapter 3 is based in effect
on the third of the above, but despite this provides a basis for predicting shape. However,
it must, because of its limitations, be deficient in predicting shape in metal complexes. We
shall see as we explore actual shapes below that it is, nevertheless, a good starting point.

4.2 Forms of Complex Life - Coordination Number and Shape

Molecules are certainly more varied than life forms. Even carbon-based compounds can
be considered as unlimited in number, despite the fact that they almost exclusively involve
four bonds around each carbon centre in a very limited number of shapes. When we move
to coordination compounds, the range of coordination numbers and shapes is expanded
considerably, so that coordination complexes live up to their name - they are inherently
complex molecular forms. Fortunately, we can identify a number of basic shapes and even
some system that governs outcomes - that is, there is some predictive aspect to shape in
coordination complexes. We shall examine complexes from the perspective of coordination
number below.

4.2.1 One Coordination (ML)

This unlikely coordination number suffers from the fact that a single donor bound to the
metal would still leave the metal highly exposed, a situation that would most likely lead to
additional ligands adding and thus increasing the coordination number. It is nevertheless
prudent to describe it as extremely rare, because there is a small possibility that a suitably
bulky and appropriately shaped ligand may achieve one-coordination. It may be more
practicable in the gas phase under high dilution conditions, where metal-ligand encounters
are limited.
As a consequence, it is not surprising that there appears to be only one isolated structure
claimed. This is of the indium(I) and thallium(I) complexes with a single M C bond from

a -bonded benzene anion that carries two bulky tri-substituted benzene substituents in

ortho positions; these partially block approach of other potential donors to the metal cation
(Figure 4.3). An X-ray crystal structure has been determined, defining the shape. It is a rare
observation, as other complexes (such as Mn(I) and Fe(I)) of the same ligand bind another
ligand at the 'open' side of the molecule, as expected. In any case, this coordination number
is trivial in the sense that it can have only one shape - a linear M L arrangement.