198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

Directional Aspects of Intermolecular Interactions 33
found, but values may reach 12. Ionic radii vary with the coordination number
[68]. For example, a cation with a tetrahedral coordination of four anions has
93–95% the radius of the same cation with a coordination number of 6, but if
the coordination number is 8, the radius is 103% that for coordination number
6. Similarly, radii for the oxide anion vary from 1.35 Å (coordination number 2),
to 1.40 Å (coordination number 6), to 1.42 Å (coordination number 8).
The directionality of metal-ion coordination can be considered in two ways –
from the point of view of a coordinating ligand such as a carboxylate or imidazole
group in a protein side chain, and from the point of view of the metal ion,
that is, how directional the bonds in its coordination sphere are. These two
aspects of metal binding are discussed here. They are relevant to the manner in
which metal ions mediate intermolecular as well as intramolecular interactions.
6.1
Metal Ion Coordination
Metal ion coordination is generally considered to consist of anions arranged
around the metal in order to fill space. Directional components of the interactions
involved are not usually taken into consideration. If, however, the cation
has a rigid coordination sphere – a regular tetrahedron for coordination number
4 or a regular octahedron for coordination number 6, and if at least two
coordination positions are filled (as, for example, by a bidentate ligand) – then
the directions of binding of the other coordination positions are well fixed. Thus
cation coordination can, in certain cases, have a strong directional component.
6.2
Magnesium Binding
Some metal ions have fairly rigid coordination spheres. A prime example is the
divalent magnesium ion which shows a great preference for six oxygen atoms
(such as from water or carboxylate groups) in its inner coordination sphere [69].
The coordination sphere can be considered rigid and if the orientation of two
ligands with respect to the magnesium ion are fixed, then the directionalities of
the other ligands are also fixed (see Fig. 25). This has been verified by studies of
a b
Fig. 25 a, b. If two locations on a rigid coordination polyhedron are fixed, the directions of
binding of the other ligands is also fixed: a 2 Mg 2+ ◊◊◊O interactions fixed in an octahedral
coordination scheme; b 2 Zn 2+ ◊◊◊S interactions fixed in a tetrahedral coordination scheme