Chemistry Reference
In-Depth Information
The molecular orbital theory recognizes that it is not essential to limit bonding between
a metal and its ligands to linkages between only two atoms at a time, but our model
developed initially dealt exclusively with what we would formally consider single (and
)
M L bonds were we applying a covalent bonding approach. So what happens if we next
introduce some double- (or
-)bond character into the M L bonds? Multiple bonds are
well known in carbon chemistry, but there is no reason that carbon should hold the patent -
and of course it doesn't, as clear from the recognition of S
O bonds, for example. In the
case of the M
L bond, at least partial double-bond character involves the consideration
of
bonding interactions of the ligand and metal
in addition to
the
bonding. There are
in effect two classes of effects,
-acceptor, relating to which way electron
density 'flows', as metal centres as well as the donor ligand can be considered as a source
of electron density.
We will briefly exemplify the
-donor and
-donor effect, by considering the interaction of metal-
based
t
2g
orbitals with filled
orbitals on a ligand. This interaction involves 'sideways'
type overlap somewhat like the simple covalent bonding concept, but multi-centred here. If
empty
t
2g
metal-based orbital interact with filled
ligand orbitals, the usual bonding and
antibonding style outcome occurs, except the resultant bonding and antibonding levels are
very close in energy to their ligand- and metal-based parents respectively. This means that
the former has dominantly the character of the ligand, and the latter that of the metal - we
can think of this as simply the metal levels being pushed up a little in energy and the ligand
orbitals down a little, through a partial transfer (or donation) of electron density from ligand
orbitals to the metal - an effect we term
-donation. There is one obvious consequence for
the metal as a result of the energy of the metal
e
g
∗
orbitals remaining unaltered - the energy
gap
o
is decreased, as seen in Figure 3.23, an effect which is reflected in the physical
properties of the complex. It is ligands such as halide or O
2
−
anions that are
-donors,
because they have available extra lone pairs after one lone pair has been used for
bonding.
These electron pairs, involved in what are in effect repulsive interactions with filled metal
orbitals, interact to lower
o
. Not surprisingly, metals with no or few d electrons tend to
e
g
*
e
g
*
E
∆
o
'
∆
o
t
2g
t
2g
π
ligand
lone pair
orbitals
ML
6
molecular
orbitals
π
-donor
interaction
Figure 3.23
The
-donor concept in metal-ligand bonding, and its influence on energy levels. Occupied lone pair
(
) orbitals on the ligand are more stable than the metal nonbonding
t
2g
orbitals. A repulsive-type
interaction between these orbitals leads to a rise (destabilization) of the
t
2g
and a fall in energy of the
ligand lone pair orbitals, and a related fall in the size of
o
.
