184
Chapter 13 “Increased-Valence” Structures for N-Centre Bonding Units
Table 13-3: Bond-lengths (Å) for 3 2
S N rings of 3 2
S N
and derivatives
39-44
.
compound
S1 – N1
S1 – N2
S2 – N1
S3 – N2
S2 – S3
3 2
S N
1.575
1.555
1.617
1.602
2.141
2
6 4
S N
1.569
1.569
1.605
1.605
2.154
3 2
S N Cl
1.617
1.543
1.581
1.615
2.136
3 2
2
S N NSO F
1.578
1.565
1.644
1.635
2.200
3 2
3
S N NCOCF
1.589
1.551
1.641
1.633
2.206
3 2
3 3 5
S N NP N F
1.573
1.540
1.647
1.528
2.220
13-8 (b) 4 4
S N : 12–electron 8–centre bonding
In Section 7-7, a molecular orbital explanation was provided for the existence of
long S-S bonds (2.58 Å
45 , cf. 2.06 Å for an S-S single-bond) in S 4 N 4 . It was
suggested that one set of nitrogen lone- pair electrons of the standard Lewis
structure (70) could delocalize appreciably into the antibonding S-S σ*-orbital,
thereby reducing the S-S bond-order below the value of unity that pertains for
structure (70). If these electrons are delocalized into the adjacent S-N bondingorbitals, “increased-valence” structure (71) is obtained
21, 38 with S-S σ-bond numbers less than unity. This structure indicates that the S-S and S-N bonds should be
respectively longer and shorter than single-bonds, and this they are found to be.
The measured S-S and S-N bond-lengths are 2.58 Å and 1.62 Å, respectively, and
Chapter 13 “Increased-Valence” Structures for N-Centre Bonding Units
Table 13-3: Bond-lengths (Å) for 3 2
S N rings of 3 2
S N
and derivatives
39-44
.
compound
S1 – N1
S1 – N2
S2 – N1
S3 – N2
S2 – S3
3 2
S N
1.575
1.555
1.617
1.602
2.141
2
6 4
S N
1.569
1.569
1.605
1.605
2.154
3 2
S N Cl
1.617
1.543
1.581
1.615
2.136
3 2
2
S N NSO F
1.578
1.565
1.644
1.635
2.200
3 2
3
S N NCOCF
1.589
1.551
1.641
1.633
2.206
3 2
3 3 5
S N NP N F
1.573
1.540
1.647
1.528
2.220
13-8 (b) 4 4
S N : 12–electron 8–centre bonding
In Section 7-7, a molecular orbital explanation was provided for the existence of
long S-S bonds (2.58 Å
45 , cf. 2.06 Å for an S-S single-bond) in S 4 N 4 . It was
suggested that one set of nitrogen lone- pair electrons of the standard Lewis
structure (70) could delocalize appreciably into the antibonding S-S σ*-orbital,
thereby reducing the S-S bond-order below the value of unity that pertains for
structure (70). If these electrons are delocalized into the adjacent S-N bondingorbitals, “increased-valence” structure (71) is obtained
21, 38 with S-S σ-bond numbers less than unity. This structure indicates that the S-S and S-N bonds should be
respectively longer and shorter than single-bonds, and this they are found to be.
The measured S-S and S-N bond-lengths are 2.58 Å and 1.62 Å, respectively, and