Chapter 4 Valence-Bond Structures for some
Diatomic and related Molecules
4-1 Molecular Orbital Configurations for Homonuclear
Diatomic Molecules
In this chapter, we shall generate valence-bond structures for some diatomic molecules and ions that involve atoms of first-row and second-row elements. To do
this, initially we shall use their molecular orbital configurations together with the
prototype valence-bond structures of Table 3-2. Green and Linnett
1 adopted this
approach to the construction of valence-bond structures, and they have described
many of the valence-bond structures that we shall consider here. We shall restrict
our attention to the molecular orbitals that are constructed from the valence-shell
2s and 2p or 3s and 3p atomic orbitals, and for simplicity in the molecular orbital
notation, neglect any hybridization that may occur between s and pσ (≡ p z ) atomic
orbitals.
In Figs. 4-1 and 4-2, we display schematic contours for the molecular orbitals
that can be constructed from 2p atomic orbitals, and the n = 2 molecular orbital
energy levels for homonuclear diatomic molecules. To construct the ground-state
molecular orbital configuration, we feed the electrons into the lowest-energy
molecular orbitals and restrict the maximum orbital occupancy to two electrons. If
only two electrons are to be allocated to a pair of degenerate molecular orbitals
(for example, the antibonding
*
x
and
*
y
molecular orbitals of 2
O ), then the
lowest-energy arrangement for these electrons occurs when each orbital is singly
occupied with parallel spins for the two electrons. In Table 4-1, we have listed the
valence-shell molecular orbital configurations for the ground states of a number of
homonuclear diatomic systems that are formed from atoms of first-row elements,
the molecular orbital bond-order (Section 3-9), and the resulting valence-bond
structure. Here, as discussed in Section 3-3, we shall use A─A as the valenceÓ 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_4
55
Diatomic and related Molecules
4-1 Molecular Orbital Configurations for Homonuclear
Diatomic Molecules
In this chapter, we shall generate valence-bond structures for some diatomic molecules and ions that involve atoms of first-row and second-row elements. To do
this, initially we shall use their molecular orbital configurations together with the
prototype valence-bond structures of Table 3-2. Green and Linnett
1 adopted this
approach to the construction of valence-bond structures, and they have described
many of the valence-bond structures that we shall consider here. We shall restrict
our attention to the molecular orbitals that are constructed from the valence-shell
2s and 2p or 3s and 3p atomic orbitals, and for simplicity in the molecular orbital
notation, neglect any hybridization that may occur between s and pσ (≡ p z ) atomic
orbitals.
In Figs. 4-1 and 4-2, we display schematic contours for the molecular orbitals
that can be constructed from 2p atomic orbitals, and the n = 2 molecular orbital
energy levels for homonuclear diatomic molecules. To construct the ground-state
molecular orbital configuration, we feed the electrons into the lowest-energy
molecular orbitals and restrict the maximum orbital occupancy to two electrons. If
only two electrons are to be allocated to a pair of degenerate molecular orbitals
(for example, the antibonding
*
x
and
*
y
molecular orbitals of 2
O ), then the
lowest-energy arrangement for these electrons occurs when each orbital is singly
occupied with parallel spins for the two electrons. In Table 4-1, we have listed the
valence-shell molecular orbital configurations for the ground states of a number of
homonuclear diatomic systems that are formed from atoms of first-row elements,
the molecular orbital bond-order (Section 3-9), and the resulting valence-bond
structure. Here, as discussed in Section 3-3, we shall use A─A as the valenceÓ 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_4
55