Living in a Cage Is a Restricted Privilege - Beauty in Chemistry - page 128

Fig. 2 How Alfred Werner assigned an octahedral structure to the two isomeric complexes of
formula [Co III (NH 3 ) 4 Cl 2 ]Cl. Only the octahedral geometry is consistent with the formation of two
isomers: the green (1,4) trans isomer and the violet (1,2) cis isomer
with respect to the other six-coordinating arrangements was later supported by
valence shell electron pair repulsion (VSEPR) theory and also by crystal field
stabilisation energy (CFSE) arguments. Indeed, Werner introduced in chemistry,
from the inorganic side, a second Platonic solid (the octahedron), which followed
the most famous and well-established polyhedron, coming from the organic
counterpart (the tetrahedron). Since then, chemists became used to “seeing”
six-coordinated metal complexes, including aquaions, arranged in the highly
symmetric octahedral structure, explaining on this geometrical basis their physical
properties and reactivity.
Over the last century, ligands containing more amine groups and displaying an
increasing degree of sophistication were synthesised and were found to impart
novel and interesting properties to the bound metal ions. Only a few chosen
transition metal ions exhibit substitutional inertness, which is related to their stable
electronic configuration, e.g. Co III and Rh III , d 6 low-spin octahedral; Cr III , d 3
octahedral; Pt II , d 8 planar, just to mention those investigated by Werner. On the
other hand, divalent 3d metal ions are labile, undergoing fast substitution processes.
Thus, their ammonia complexes cannot be isolated in different isomeric forms,
as can those of Co III . Nevertheless, they presented interesting properties, which
allowed the introduction of new concepts in chemistry. In particular, the coordina-
tion chemistry of multidentate ligands developed, i.e. molecules containing more
amine groups (Fig. 3 ), also defined as chelating agents. It was observed that a given
metal complex of an n -dentate ligand was thermodynamically more stable than the
corresponding complex with n molecules of NH 3 (the chelate effect) and that such
an extra stability increased with the ligand's denticity [ 3 ]. Later, comparison of the
solution behaviour of the divalent 3d metal complexes of aliphatic tetra-amines,
whether open-chain or cyclic, disclosed further interesting properties. In particular,
the cyclic tetramine ligand (the macrocycle) forms a more stable complex (by three
orders of magnitude or more) with a given metal than its non-cyclic counterpart
(the so called thermodynamic macrocyclic effect) [ 4 ].
As an example, Fig. 4 illustrates the Ni II complexation equilibria in water
involving 2.3.2-tet (1), champion of non-cyclic tetra-amines, and cyclam (2),