256
Chapter 19 Some Electron-Excess σ Bonded Systems
Maclagan
20 has discussed the bonding for
6
SF , and has concluded that ionic
Lewis structures such as structure (48), which also involve an expansion of the
sulphur valence shell, should have larger weights than has structure (47). Each of
these ionic structures can be stabilized by delocalizing a non-bonding 2pσ-electron
from each of the F
 into a bonding S-F σ-orbital to generate “increased-valence”
structure (49), which is more stable than structure (48). “Increased-valence”
structure (49) has an expanded valence shell, and a 4-electron 3-centre bonding
unit with a Pauling “3-electron bond” as a component. The bond-lengths for 6
SF
suggest that resonance between “increased-valence” structures of type (49),
together with some contribution from structure (47), provides a more suitable
representation of the electronic structure than does resonance between “increasedvalence” structures of type (46). In structures (47) and (49), the sulphur hybridizations are
3 2
sp d and
3
sp d , respectively, which involve with the sulphur g
e type

3d orbitals.
Studies
21 of the geometry for 5
SF Cl give
ax
S F 1.571


Å,
ax
S Cl
2.055


Å
and
eq
S F
1.570


Å, which are similar to the earlier reported lengths of 1.588,
2.047 and 1.566 Å. Resonance between valence-bond structures similar to
structures (47) and (49) accounts for the observed bond lengthenings relative to
the estimates of 1.54 and 2.01 Å for S-F and S-Cl single bonds.
19-5 Thiothiophthenes
Bond lengths
22 for thiothiophthene are reported in (50).
The S-S bonds for this molecule and many of its derivatives
22, 23 are
appreciably longer than the standard single-bond length of 2.06 Å. (For the deriva-