Chemistry Reference
In-Depth Information
is invariably larger for 4d and 5d elements versus 3d elements, nor can it provide a really
satisfactory interpretation of the variation of
o
with ligand type. While there is an array
of experimental outcomes from a range of methods that can be, at least qualitatively, fitted
to the d-orbital splitting model of CFT, quantitative calculations based on this electrostatic
model tend to fail to match experimental values. However, the key failing is that a purely
ionic description of bonding in coordination complexes is not consistent with the now
abundant experimental evidence. Put simply, electrons in d orbitals, which are fully located
therein in the CFT model, in fact spend part of their time in ligand orbitals (they are
'delocalised', or there is covalency in the bonding). The spectroscopic method of electron
spin resonance, which 'maps' unpaired electron density, indicates that 'pure' d orbitals in
fact can have their electrons appreciably delocalized over the ligands, whereas evidence
for donation of ligand electron density to the metal comes from the nephelauxetic effect
of electronic spectroscopy. (The term 'nephelauxetic' means 'cloud expanding', and the
effect relates to the observation that electron pairing energies are lower in metal complexes
than in 'naked' gaseous metal ions. This suggests that interelectronic repulsion has fallen
on complexation, which equates with the effective size of metal orbitals increasing. The
effect varies with the type of ligand bound to the metal.)
This outcome can hardly be a surprise, since partial covalency has also been invoked to
account for the properties of simple ionic solids. Moreover, in the same way that we do
not need to throw out the ionic bonding model of the solid state to accommodate these
observations, we do not need to reject the CFT wholesale. In particular, the simple orbital
splitting model for the five d orbitals can continue to provide useful service, but amendment
to accommodate covalency is necessary. This led to the development of ligand field theory
(LFT), which, as the name implies, recognizes the role of the ligand more effectively. While
CFT and LFT tend to be used interchangeably, they differ in that the former is a classical
electrostatic model whereas the latter is more a MO model. As we have seen earlier, LFT
leads to metal-centred MOs that correspond to the d-orbital energy levels from CFT, and
hence it has become common to use LFT as an encompassing theory that includes elements
of CFT, particularly where the focus is on the five d orbitals (Figure 3.14). However, all of
our discussion to date has focused on one, albeit common, geometry - octahedral.
One outcome of the earlier discussion, which identified the wealth of shapes adopted by
metal complexes, is a recognition that the bonding models we introduced above exclusively
for six-coordinate octahedral complexes are clearly limited if they are restricted to this one
shape. That different shaped complexes exist is a fact, so our model developed to date must
be flexible enough to accommodate other shapes. One of the best ways to understand how
this works is to start with our octahedral field, and look at what happens as we distort it.
One well known type of structural distortion is where there is one unique axis where the
bond distances are longer than along the other two axes; it still has the basic octahedral
shape, but is 'stretched' along one axis direction - like a stretched limo still being, at heart,
a standard car. In the extreme, these stretched bonds get so long that they effectively don't
exist, meaning that there are only four bonds left, in a plane around the central metal. By
convention, the 'stretched' axis is defined as the
z
axis. As such, it is the orbitals which
point in the
z
direction that feel the effect of change most - easy to identify, from their
names, as d
z
2
,d
xz
and d
yz
. As we pull the two point charges away from the metal, their
influence on these three orbitals diminishes, and the orbitals fall in relative energy (or are
stabilized); this removes the degeneracy from both the diagonal set
and
the axial set of
orbitals, as at least one of the d
z
2
,d
xz
and d
yz
orbital resides in each set. The outcome is
shown in Figure 3.15. This trend continues to the extreme case where they are removed
