Chemistry Reference
In-Depth Information
and also leads to an O
2
bond order of 2. The 2
π
g
level is also doubly degenerate, and so
the two electrons can reside in different spatial orbitals with their spins aligned, making
O
2
paramagnetic.
In the PES of O
2
the 5
σ
g
+
band is split in two (Figure 7.30). This results from the
two alternative spin states for the O
2
+
cation that is formed on ionization; referring to
Figure 7.31, removing the spin-down electron from 5
σ
g
+
will give a cation with three
unpaired spins, while removing the spin-up electron will give a cation with only one
unpaired spin. The difference in the energies of these alternative cation spin states leads to
the observed splitting. A similar effect does occur for ionization from the 1
π
u
, but the split-
ting is smaller and it is masked by the broad spread of the vibrational peaks for ionization
from this orbital.
The third spectrum in Figure 7.30 is for F
2
; this has the same ordering of peaks as O
2
but
considerably more spread out. Now the antibonding 2
π
g
state contains four electrons, so
that the bond order is 1 and the molecule is diamagnetic. The first ionization of F
2
requires
more energy than O
2
because of the higher effective core charge of the F atoms.
The PESs show that sp-hybridization is important for the diatomics from Li
2
up to N
2
.
So we can predict the ground-state electronic configurations of the unstable diatomics from
the early part of the second row:
σ
g
+
)
2
Li
2
:(3
A single bond.
σ
g
+
)
2
,(4
σ
u
+
)
2
Be
2
:(3
Formally no net bonding, although the sp-hybrid
scheme does suggest a weak interaction because both
3
σ
g
+
and 4
σ
u
+
are lowered by hybridization.
B
2
:(3
σ
g
+
)
2
,(4
σ
u
+
)
2
,(1
π
u
)
2
A single bond. This would also give a triplet ground
state because the two electrons in the 1
π
u
level occupy
different orbitals.
C
2
:(3
σ
g
+
)
2
,(4
σ
u
+
)
2
,(1
π
u
)
4
A double bond with no unpaired electrons.
Table 7.3 does list data for Li
2
,B
2
and C
2
, but, as expected from above, Be
2
is very
difficult to observe experimentally. Li
2
has a weak single bond with an estimated energy
of 106 kJ mol
−
1
. This is much lower than the bond energy of H
2
because of the larger radius
of the 2s orbitals compared with 1s and the repulsion between the core states on the two
Li atoms. This also leads to a very long bond length.
The single bond of B
2
is considerably stronger; as we have seen, the 4
σ
u
+
antibonding
character is reduced by hybridization. The bond energy of C
2
is the highest in the set
of these 'unusual' diatomics. In fact, B
2
and C
2
both have bond energies considerably
higher than that of the more familiar F
2
molecule. However, under normal conditions, the
electron-deficient nature of these elements, early in the second row, leads to a preference
for forming metallic solids (Li, Be) or insulating solids consisting of extended covalent
arrays (B, C).
7.5.2 Heteronuclear Diatomics of Second-Row Elements
When the two atoms in a diatomic are different, the molecule belongs to the
C
∞
v
point
group. There are several important diatomics from second-row elements; CO and NO,
for example, are important in inorganic chemistry as ligands in inorganic complexes. The
strong binding of CO to metal centres also makes it capable of inhibiting the Fe(haem)
