Chemistry Reference
In-Depth Information
Fig. 11
Top
: Structures of metal complexes of sarcophagines, as obtained from X-ray diffraction
studies [
25
-
28
]; Cr
III
is complexed by (NH
2
)
2
sar; all other metals are complexed by (NH
3
)
2
sar
2+
.
Bottom
: Top view of the triangles obtained by joining the nitrogen atoms of the secondary amine
groups linked to each one of the two caps of sarcophagine ligands, with the values of torsion
angles
y
geometry and its [Mn
II
(NH
3
)
2
sar]
4+
complex shows a torsion value
0.3
[
28
]. This is the same value observed for the Mg
II
complex, which corresponds to the
less-strained conformational arrangement of the coordinated sarcophagine,
y
ΒΌ
27.7
in
the absence of any crystal field effect.
3 Cryptands and Cryptates: Cages Made to Measure
for s-Block Metals
Coordination chemistry was born with transition metals, a circumstance that
reflects the relatively high energy of the metal-ligand interaction. The
sp
3
amine
nitrogen atom is the deputed donor for establishing strong s coordinative
interactions and ammonia is the prototype ligand for transition metal ions. On the
other hand, for s-block metal ions, it is the
sp
3
oxygen that is the donor atom
displaying the highest affinity and water is expected to be the prototype ligand.
Indeed, alkali and alkaline earth metal salts dissolve in water, where the metal is
surrounded by a shell of coordinating solvent molecules. However, such aqua-
complexes are not stable enough to be isolated and, on crystallisation, salts are
formed in which the metal ion directly interacts (electrostatically) with the anion.
Transition metal coordination chemistry has taught some tricks for enhancing
the stability of complexes, for instance making use of multidentate ligands. How-
ever, the intrinsic stability of the complexes of s-block metal ions with ligands
containing
sp
3
oxygen donor atoms is so low that the simple chelate effect fails: in
fact, poorly stable complexes are formed with glycols, the analogues of polyamines.
But, if the chelate effect does not work, the macrocyclic effect does. This was
