100
Chapter 7 Some Dimers of Triatomic Radicals with 17 and 19 Valence-Shell Electrons
weights than do those for
2
2
4
S O
; the monomer odd-electron charges (Table 6-1)
give weights for these structures of 0.19, 0.19 and 0.03 for
2
2
4
S O
and 0.08, 0.00
and 0.00 for P 2 F 4 . The staggered trans geometries for P 2 F 4 and P 2 H 4 are due
primarily to the non-bonded repulsions between the phosphorus lone-pair
electrons (Section 3-10) and repulsions between net charges on the fluorine and
hydrogen atoms. Similar effects for
2
2
4
S O
might be overcome by covalent-ionic
resonance of the type (21) ↔ (29), (cf. (4) ↔ (10) for N 2 O 4 ), which could
introduce a significant cis O-O overlap stabilization energy. Due to the much
smaller weight for covalent eclipsed (or staggered) structures of type (30)
for P 2 F 4 , a cis F-F overlap stabilization energy through the resonance (30) ↔ (31)
will be of negligible importance for this molecule, and cannot operate for P 2 H 4 .
Similar theory accounts for the existence of trans and gauche rather than eclipsed
conformers for N 2 F 4 .
It has been suggested
25 that structures of type (22) should help stabilize the
eclipsed conformation for
2
2
4
S O
. However, as is the case for structure (6) for
N 2 O 4 , the cis O-O bond of structure (22) must have negligible strength; its bondlength is 2.86 Å.
7-7 C-Nitroso Dimers and S 4 N 4
In their standard Lewis structures (1) and (19), both N 2 O 4 and
2
2
4
S O
carry
positive formal charges on the adjacent nitrogen and sulphur atoms. The presence
of such charges helps to induce considerable delocalization of oxygen lone-pair
electrons into an antibonding
*
-orbital between the nitrogen or sulphur atoms,
with a consequent lengthening of the N-N and S-S bonds relative to single-bond
Chapter 7 Some Dimers of Triatomic Radicals with 17 and 19 Valence-Shell Electrons
weights than do those for
2
2
4
S O
; the monomer odd-electron charges (Table 6-1)
give weights for these structures of 0.19, 0.19 and 0.03 for
2
2
4
S O
and 0.08, 0.00
and 0.00 for P 2 F 4 . The staggered trans geometries for P 2 F 4 and P 2 H 4 are due
primarily to the non-bonded repulsions between the phosphorus lone-pair
electrons (Section 3-10) and repulsions between net charges on the fluorine and
hydrogen atoms. Similar effects for
2
2
4
S O
might be overcome by covalent-ionic
resonance of the type (21) ↔ (29), (cf. (4) ↔ (10) for N 2 O 4 ), which could
introduce a significant cis O-O overlap stabilization energy. Due to the much
smaller weight for covalent eclipsed (or staggered) structures of type (30)
for P 2 F 4 , a cis F-F overlap stabilization energy through the resonance (30) ↔ (31)
will be of negligible importance for this molecule, and cannot operate for P 2 H 4 .
Similar theory accounts for the existence of trans and gauche rather than eclipsed
conformers for N 2 F 4 .
It has been suggested
25 that structures of type (22) should help stabilize the
eclipsed conformation for
2
2
4
S O
. However, as is the case for structure (6) for
N 2 O 4 , the cis O-O bond of structure (22) must have negligible strength; its bondlength is 2.86 Å.
7-7 C-Nitroso Dimers and S 4 N 4
In their standard Lewis structures (1) and (19), both N 2 O 4 and
2
2
4
S O
carry
positive formal charges on the adjacent nitrogen and sulphur atoms. The presence
of such charges helps to induce considerable delocalization of oxygen lone-pair
electrons into an antibonding
*
-orbital between the nitrogen or sulphur atoms,
with a consequent lengthening of the N-N and S-S bonds relative to single-bond