13-3
Comments on 6-Electron 4-Centre Bonding Units
171
Table 13-2: Bond-lengths for dimeric nitroso and structurally similar compounds
13
.
bond length Å
a
compound names
C-N
N-N
N-O
geometry
Ref
e
cis-Azobenzene dioxide
1.454 (5)
1.321 (5) 1.268 (4)
cis
present
work
(Nitrosobenzene dimer)
1.463 (5)
1.261 (4)
trans-2.2-Dicarboxyazobenzene dioxide
1.460 (3)
1.308 (3) 1.267 (3)
trans
present
work
(2-Nitrosobenzoic acid
dimer) Perfluoroazobenzene
dioxide
1.439 (6)
1.324 (5) 1.267 (6)
cis
41
(Pentafluoronitrosobenzene
dimer)
1.439 (6)
1.267 (6)
1, 8-Dinitrosonaphthalene
1.430 (6)
1.376 (5) 1.276 (6)
cis
25
(Internal dimer)
1.439 (6)
1.256 (5)
Nitrosocyclohexane
1.488 (6)
1.319 (6) 1.272 (6)
trans
19
2-Nitronitrosoethane
1.470 (4)
b
1.304 (6) 1.262 (4)
c
trans
18
Azoxyanisole
1.496 (5)
1.218 (5) 1.279 (4)
trans
42
Azobenzene
d
1.433 (3)
1.243 (3)
trans
43
p.p′-Dichloroazobenzene
1.433 (5)
1.252 (5)
trans
44
a
Estimated standard deviations are given in parentheses.
b
The C-N bond involving the nitroso group.
c
The N-O bond of the nitroso group.
d
Two unique molecules are present in the unit cell, one of which is disordered. The data
presented are from the nondisordered molecule. The estimated standard deviations may be
severely underestimated.
e
See Ref. 13 for details of these references.
13-3 Comments on 6-Electron 4-Centre Bonding Units
Two “increased-valence” bonding units of the general type (7) are present in the
“increased-valence” structures of (6); one for the six π-electrons and one for six
electrons
The relevant atomic orbitals are displayed in Fig. 13-2 for the cis
isomer. The latter type of “increased-valence” bonding unit is also present in
structures (2) and (4) for the six
electrons
of
2
2 4
C O
and 2 4
N O . In this
chapter and the previous chapters, we have introduced two techniques that may be
used to generate (7), namely
(i) To bond together two Pauling “3-electron bond” structures
A · B and
C · D
(with antiparallel spins for the two antibonding odd-electrons), and
Comments on 6-Electron 4-Centre Bonding Units
171
Table 13-2: Bond-lengths for dimeric nitroso and structurally similar compounds
13
.
bond length Å
a
compound names
C-N
N-N
N-O
geometry
Ref
e
cis-Azobenzene dioxide
1.454 (5)
1.321 (5) 1.268 (4)
cis
present
work
(Nitrosobenzene dimer)
1.463 (5)
1.261 (4)
trans-2.2-Dicarboxyazobenzene dioxide
1.460 (3)
1.308 (3) 1.267 (3)
trans
present
work
(2-Nitrosobenzoic acid
dimer) Perfluoroazobenzene
dioxide
1.439 (6)
1.324 (5) 1.267 (6)
cis
41
(Pentafluoronitrosobenzene
dimer)
1.439 (6)
1.267 (6)
1, 8-Dinitrosonaphthalene
1.430 (6)
1.376 (5) 1.276 (6)
cis
25
(Internal dimer)
1.439 (6)
1.256 (5)
Nitrosocyclohexane
1.488 (6)
1.319 (6) 1.272 (6)
trans
19
2-Nitronitrosoethane
1.470 (4)
b
1.304 (6) 1.262 (4)
c
trans
18
Azoxyanisole
1.496 (5)
1.218 (5) 1.279 (4)
trans
42
Azobenzene
d
1.433 (3)
1.243 (3)
trans
43
p.p′-Dichloroazobenzene
1.433 (5)
1.252 (5)
trans
44
a
Estimated standard deviations are given in parentheses.
b
The C-N bond involving the nitroso group.
c
The N-O bond of the nitroso group.
d
Two unique molecules are present in the unit cell, one of which is disordered. The data
presented are from the nondisordered molecule. The estimated standard deviations may be
severely underestimated.
e
See Ref. 13 for details of these references.
13-3 Comments on 6-Electron 4-Centre Bonding Units
Two “increased-valence” bonding units of the general type (7) are present in the
“increased-valence” structures of (6); one for the six π-electrons and one for six
electrons
The relevant atomic orbitals are displayed in Fig. 13-2 for the cis
isomer. The latter type of “increased-valence” bonding unit is also present in
structures (2) and (4) for the six
electrons
of
2
2 4
C O
and 2 4
N O . In this
chapter and the previous chapters, we have introduced two techniques that may be
used to generate (7), namely
(i) To bond together two Pauling “3-electron bond” structures
A · B and
C · D
(with antiparallel spins for the two antibonding odd-electrons), and