220
Chapter 17 Some Tetrahedral Molecules
are examples of Lewis octet and Lewis expanded valence-shell structures. The
“increased-valence” structure
i† (3) does not involve the participation of sulphur
3d-orbitals as hybridization functions in bonding.
Structures (2) and (3) are obtained respectively by delocalizing oxygen π or/and
electrons of structure (1) into either a vacant sulphur 3d-orbital or into bonding
S-O orbitals; the relevant (overlapping) atomic orbitals for the latter are a sulphur
hybrid orbital of an adjacent S-C σ-bond and an oxygen π- or -orbital. In
structure (3), 1-electron bonds replace the additional d π – p π bond that is formed
when the sulphur atom expands its valence shell in structure (2). Structures (4)-(6)
are similar types of structures for
2
4
SO
, with the sulphur atom for structure (5)
utilizing two 3d-orbitals to form the two d π – p π S-O bonds.
It is important to distinguish between the role of d-orbitals as hybridization
functions and their role as polarization functions. For the former case, the d-orbital
serves to “increase the number of distinct orbitals utilized in the wave-function”
1
.
Thus, the octet and expanded valence-shell structures (1) and (2) have C 1 s
2 p
3 ,
4
3
2 sp
V C
, 3
V and
2 3
1 s p d
C
,
4
4
2 sp d
V C
, 4
V valence-state configurations for the
trivalent and quadrivalent sulphur atoms. For structures (4) and (5) the sulphur
valence-state configurations are
3
sp , 4
V and
3 2
sp d , 6
V . When a 3d-orbital
participates as a polarization function, it “merely moderates the shape of preexisting orthogonal hybrid atomic orbitals”
1 . For example, the three sulphur
orbitals that form σ-bonds in the octet structure (1) may involve some 3d as well
as 3s and 3p character; i.e. each orbital may be expressed as 1
2
3
s
p
d
c c
c
. The
resulting valence-state configuration is then
2 3
1 s p
C
,
4
3
2 sp
V C
,
2 2
3
3 s p d
V C
,
3
3
4 sp d
V C
, 3
V . Therefore, the utilization of 3d orbitals as polarization functions
is not precluded for either the octet structures or “increased-valence” structures
such as (3) and (6). Our concern here is to distinguish between this type of
“increased-valence” structure, and those that utilize 3d orbitals as hybridization
functions for d π – p π bonding. We shall restrict our attention primarily to a consideration of the bonding for some sulphur compounds.
On the basis of some bond-length data, we shall suggest that expansion of the
sulphur valence-shell to generate d π – p π bonding is more likely to occur in either
of the following situations:
(a) The R substituents of R 2 SO are alkyl radicals. In Sections 11-3 and 11-4 we
have concluded that H and alkyl substituents seem unable to stabilize appreciable development of Pauling “3-electron bonds” in 4-electron 3-centre bonding units of neutral molecules. The delocalization of oxygen or electrons
i Relative to the octet Lewis octet structures (1) and (4), structures (2) and (5) as well as
structures (3) and (6) exhibit “increased-valence”, i.e. more electrons participate in bonding
than does occur in structures (1) and (4). In this Chapter (and throughout this book), we refer
to valence bond structures that involve one or more Pauling “3-electron bonds” as “increasedvalence” structures.
Chapter 17 Some Tetrahedral Molecules
are examples of Lewis octet and Lewis expanded valence-shell structures. The
“increased-valence” structure
i† (3) does not involve the participation of sulphur
3d-orbitals as hybridization functions in bonding.
Structures (2) and (3) are obtained respectively by delocalizing oxygen π or/and
electrons of structure (1) into either a vacant sulphur 3d-orbital or into bonding
S-O orbitals; the relevant (overlapping) atomic orbitals for the latter are a sulphur
hybrid orbital of an adjacent S-C σ-bond and an oxygen π- or -orbital. In
structure (3), 1-electron bonds replace the additional d π – p π bond that is formed
when the sulphur atom expands its valence shell in structure (2). Structures (4)-(6)
are similar types of structures for
2
4
SO
, with the sulphur atom for structure (5)
utilizing two 3d-orbitals to form the two d π – p π S-O bonds.
It is important to distinguish between the role of d-orbitals as hybridization
functions and their role as polarization functions. For the former case, the d-orbital
serves to “increase the number of distinct orbitals utilized in the wave-function”
1
.
Thus, the octet and expanded valence-shell structures (1) and (2) have C 1 s
2 p
3 ,
4
3
2 sp
V C
, 3
V and
2 3
1 s p d
C
,
4
4
2 sp d
V C
, 4
V valence-state configurations for the
trivalent and quadrivalent sulphur atoms. For structures (4) and (5) the sulphur
valence-state configurations are
3
sp , 4
V and
3 2
sp d , 6
V . When a 3d-orbital
participates as a polarization function, it “merely moderates the shape of preexisting orthogonal hybrid atomic orbitals”
1 . For example, the three sulphur
orbitals that form σ-bonds in the octet structure (1) may involve some 3d as well
as 3s and 3p character; i.e. each orbital may be expressed as 1
2
3
s
p
d
c c
c
. The
resulting valence-state configuration is then
2 3
1 s p
C
,
4
3
2 sp
V C
,
2 2
3
3 s p d
V C
,
3
3
4 sp d
V C
, 3
V . Therefore, the utilization of 3d orbitals as polarization functions
is not precluded for either the octet structures or “increased-valence” structures
such as (3) and (6). Our concern here is to distinguish between this type of
“increased-valence” structure, and those that utilize 3d orbitals as hybridization
functions for d π – p π bonding. We shall restrict our attention primarily to a consideration of the bonding for some sulphur compounds.
On the basis of some bond-length data, we shall suggest that expansion of the
sulphur valence-shell to generate d π – p π bonding is more likely to occur in either
of the following situations:
(a) The R substituents of R 2 SO are alkyl radicals. In Sections 11-3 and 11-4 we
have concluded that H and alkyl substituents seem unable to stabilize appreciable development of Pauling “3-electron bonds” in 4-electron 3-centre bonding units of neutral molecules. The delocalization of oxygen or electrons
i Relative to the octet Lewis octet structures (1) and (4), structures (2) and (5) as well as
structures (3) and (6) exhibit “increased-valence”, i.e. more electrons participate in bonding
than does occur in structures (1) and (4). In this Chapter (and throughout this book), we refer
to valence bond structures that involve one or more Pauling “3-electron bonds” as “increasedvalence” structures.