Chapter 19 Some Electron-Excess σ Bonded
Systems
Most of the “increased-valence” structures that we have discussed so far may be
derived from Lewis structures by delocalizing lone-pair π and/or electrons into
vacant bonding or antibonding orbitals. The atomic orbital overlaps that are
appropriate for some of these delocalizations are shown in Figs. 1-5 and 2-4. We
shall now consider a few systems whose “increased-valence” structures can be
constructed by delocalizing one or more lone-pair σ electrons of a Lewis structure
into bonding or antibonding σ orbitals. Some other examples will also be
discussed in Chapter 20, where the theory will be presented in a slightly different
form. However, the principles for both chapters are the same.
19-1 Trihalide Anions and some Related Molecules
Each of the trihalide anions 3
I
, 3
Br
, 3
Cl
and
3
ICl
, and
2
XeF , has 22 valenceshell electrons.
2
XeF is a symmetrical linear molecule
1 , and a similar geometry
has been reported for each of 3
I
, 3
Br
and
2
ICl
2 . Non-symmetrical geometries
for some of these trihalide ions are also known
2
, but we shall not concern
ourselves with them here. Excluding the possibility of d-orbital participation, we
shall now describe standard Lewis, “increased-valence” and Linnett (Section 2-2)
non-paired spatial orbital bonding schemes for these systems, using 3
I
and
2
XeF
as representative examples. Such molecules are often designated as geometrical
“hypervalent” molecules, for which the number of ligands bonded to a central
atom exceeds the covalence of the central atom in the standard Lewis octet
structures. Musher
3 has discussed various examples of hypervalent molecules. We
point out here that geometric hypervalence for an atom A may arise whenever
valence-bond resonance of the type
Ó Springer International Publishing Switzerland 2016
R.D. Harcourt, Bonding in Electron-Rich Molecules,
Lecture Notes in Chemistry 90, DOI 10.1007/978-3-319-16676-6_19
247
Systems
Most of the “increased-valence” structures that we have discussed so far may be
derived from Lewis structures by delocalizing lone-pair π and/or electrons into
vacant bonding or antibonding orbitals. The atomic orbital overlaps that are
appropriate for some of these delocalizations are shown in Figs. 1-5 and 2-4. We
shall now consider a few systems whose “increased-valence” structures can be
constructed by delocalizing one or more lone-pair σ electrons of a Lewis structure
into bonding or antibonding σ orbitals. Some other examples will also be
discussed in Chapter 20, where the theory will be presented in a slightly different
form. However, the principles for both chapters are the same.
19-1 Trihalide Anions and some Related Molecules
Each of the trihalide anions 3
I
, 3
Br
, 3
Cl
and
3
ICl
, and
2
XeF , has 22 valenceshell electrons.
2
XeF is a symmetrical linear molecule
1 , and a similar geometry
has been reported for each of 3
I
, 3
Br
and
2
ICl
2 . Non-symmetrical geometries
for some of these trihalide ions are also known
2
, but we shall not concern
ourselves with them here. Excluding the possibility of d-orbital participation, we
shall now describe standard Lewis, “increased-valence” and Linnett (Section 2-2)
non-paired spatial orbital bonding schemes for these systems, using 3
I
and
2
XeF
as representative examples. Such molecules are often designated as geometrical
“hypervalent” molecules, for which the number of ligands bonded to a central
atom exceeds the covalence of the central atom in the standard Lewis octet
structures. Musher
3 has discussed various examples of hypervalent molecules. We
point out here that geometric hypervalence for an atom A may arise whenever
valence-bond resonance of the type
Ó Springer International Publishing Switzerland 2016
R.D. Harcourt, Bonding in Electron-Rich Molecules,
Lecture Notes in Chemistry 90, DOI 10.1007/978-3-319-16676-6_19
247