Chemistry Reference
In-Depth Information
Merriam-Webster dictionary online as “an arrangement of atoms or molecules so
bonded as to enclose a space in which another atom or ion (as of a metal) can
reside” [
1
]. We can add that a caged chemical can profit from some special
properties that are not granted to non-caged counterparts, which makes living in a
molecular cage a notable privilege, as we will try to demonstrate.
2 From Ammonia to Sarcophagine: Cages for Metals
Made by Metals
Transition metal ions are among the most demanding guests of a cage. They engage
in bonding
d
orbitals, which possess well-defined directional properties and,
depending upon their electronic configuration, they discriminate between different
coordination geometries in order to profit to the largest extent from ligand field
stabilisation effects. The most common coordination geometry is the octahedral one
and the electronic configuration that profits most from octahedral geometry (24 Dq,
in the language of Crystal Field Theory) is
d
6
low-spin. It is not accidental that
coordination chemistry began with the synthesis and characterisation of octahedral
complexes of Co
III
(
d
6
low-spin) with the most common and cheapest ligand:
ammonia. The founder of coordination chemistry, Alfred Werner (1866-1919),
thanks to the substitutional inertness of the Co
III
centre, a property related to the
stable electronic configuration, isolated a variety of cobalt ammonia complexes,
which could be characterised by elemental analysis and simple reactivity tests.
Even in the absence any direct physico-chemical technique (e.g. single crystal
X-ray diffraction), Werner was able to induce not only stoichiometry, but also the
geometrical structure of the investigated complexes [
2
]. The most famous example
refers to the complex of formula Co(NH
3
)
4
Cl
3
, which was isolated in two isomeric
forms of distinct colour: green and violet. Addition of AgNO
3
to the solutions of the
two complexes, at room temperature, caused in both cases the precipitation of
1 equiv. of AgCl, which indicated that only two Cl
ions were directly bound to
the metal, to which the following formula corresponded: [Co
III
(NH
3
)
4
Cl
2
]
+
. Then,
Werner considered that, for coordination number 6, three coordination geometries
were possible: hexagonal, trigonal prismatic and octahedral. Of the three geometri-
cal arrangements, the hexagon and the trigonal prism would give rise to three
isomers, as shown in Fig.
2
. Thus, the correct geometry had to be the octahedral
one, whose expected isomers are just two. Professor Werner was pretty sure of his
ability at the bench for not missing a “third” isomer (but he had investigated 20
series of compounds showing this type of isomerism). Also the “configuration
allocation”, i.e. the discrimination of
cis
and
trans
isomers was carried out on a
chemical basis: Werner noted that on substitution by two Cl
ions of the bound
bidentate carbonate ion in the complex [Co
III
(NH
3
)
4
CO
3
]
+
, which had to necessar-
ily exhibit a
cis
configuration, the violet form was obtained, to which the
cis
configuration could be unequivocally assigned. The advantage of the octahedron
