Chemistry Reference
In-Depth Information
ψ
−
BN
E
(antibonding)
ψ
BF
3
ψ
NH
3
ψ
+
BN
(bonding)
BF
3
LUMO
NH
3
HOMO
.
F
3
B
NH
3
Figure 3.10
A molecular orbital diagram for F
3
B NH
3
, reporting only the orbital interaction of the lowest
unoccupied molecular orbital (LUMO) on the precursor molecule BF
3
and the highest occupied
molecular orbital (HOMO) on the precursor molecule NH
3
. Only if there is useful overlap of the
orbital contributions from each precursor are the new orbitals formed.
originally in the N orbital occupy what we can consider as the bonding MO, leading to
a more stable situation for the B N assembly (the electrons occupying a lower energy
orbital compared with the energy of the orbital on the original N centre), and hence a stable
bonding outcome. In more complicated metal complexes, it is again convenient to consider
just the one relevant orbital on each donor molecule or atom (effectively the lone pair in the
valence bond description) together with the valence orbitals on the central metal in building
the MO diagram.
In our ML
6
model, there were originally nine orbitals of similar energy on the metal
considered (five d, one s and three p), yet to form bonds to six ligand orbitals only six are
required. This means that three are not involved in forming bonds to the ligands, and are
thus designated as nonbonding orbitals (Figure 3.11). While it is not necessarily a good idea
to always seek linkages between models, these nonbonding levels effectively correspond
to the
t
2g
set of the ionic model already discussed. Overall, 15 'orbitals of interest' appear
in the diagram, capable of accommodating up to 30 electrons. Like all MO models, the
number of participating orbitals in the components (the metal and six ligand orbitals here)
must equal the number of MOs in the assembly (ML
6
here). Moreover, equal numbers of
bonding and antibonding orbitals, the former of lower energy and the latter of higher energy
than the parent orbitals, must form.
The 12 electrons from the six ligands lone pairs are used to occupy the six bonding
molecular
-orbitals created (defined by their symmetry labels as
a
1g
(singlet),
t
1u
(triplet),
and
e
g
(doublet)), which lie lower but fairly close in energy to the initial ligand orbital
energy levels, with the metal d electrons then inserted in the next highest energy levels (
t
2g
,
e
g
∗
) of the molecular assembly. This means that insertion of d electrons commences in the
model at the nonbonding orbitals derived from the original d-orbital set. This set of three
nonbonding levels along with the six bonding orbitals provide an upper limit of 18 electrons
before antibonding orbitals must be employed, which would then lead to an inherently
reduced stability for the system. Moreover, there are some benefits for some types of
complexes in having this nonbonding set filled fully, with filling leading to some favourable
