Chemistry Reference
In-Depth Information
R
-
O
-
R
N
O
N
Ni
II
Ni
II
-
O
-
O
N
N
R
R
square planar
tetrahedral
Figure 4.12
An N,O-chelate ligand whose Ni(II) complex displays interconversion between different geometries
depending on conditions.
in that case simply from combination of the colours of the two independent ions. When
metal ions come very close together, this interaction will also affect the magnetic properties
of their assembly significantly.
In describing the relationship between tetrahedral and square planar complexes above,
we used a model where interconversion could occur without bond breaking. If this is a
fair representation of reality, then it should be possible to find some systems that exist
either as a mixture of the two forms in equilibrium or can convert between the two forms
when a change in conditions is applied. Fortunately, there are indeed some compounds that
undergo conversion between tetrahedral and square planar forms in solution, a situation
which implies that the stabilities of the two forms are very similar. One now classical
example involved the Ni(II) complex of a chelated N,O-donor ligand shown in Figure 4.12,
where conversion between dominantly tetrahedral and dominantly square planar forms
depends on the temperature, solvent and the type of R-group attached to the coordinated
imine nitrogen. Change between the two forms can be readily monitored, since their colours
and absorption band positions and intensities are different.
4.2.5
Five Coordination (ML
5
)
Examples of ML
5
are found for all of the first row transition metal ions, as well as some
other metal ions. Although once considered rare, growth in coordination chemistry has led
to five-coordination becoming met almost as frequently as four-coordination. This exhibits
one of the limitations of making comparisons of this type; rarity may not be a result of
any inherent restriction, but may simply reflect limited experimental development. Given
that four-coordination is common and six-coordination very common, it is perhaps not
surprising to find five-coordination also having matching status, at least for lighter, smaller
metal ions. Five-coordination is not commonly met in complexes of the heavier transition
metals, however.
The amended VSEPR model predicts two forms of five-coordination, and experimental
chemistry has clearly identified many examples of both forms. These limiting structures are
square-based pyramidal (or, simply, square pyramidal) and trigonal bipyramidal (Figure
4.13). The classical square-based pyramidal shape is formed simply by cleaving off one
bond from an octahedral shape, which leaves the metal in the same plane as the four square-
based ligands. In reality, almost no complexes exhibit this shape, but rather adopt a distorted
