Chemistry Reference
In-Depth Information
7
Symmetry in Chemical Bonding
7.1
Introduction
In earlier chapters we classified the symmetry of atomic orbitals (AOs) in a number of
example molecules. It is now time to develop the ideas of molecular orbital (MO) theory
and use it to describe chemical bonding. Symmetry classifications help in the MO descrip-
tion of chemical bonding because symmetry controls how the AOs on neighbouring atoms
mix together. MOs are the wavefunctions for electrons in the complex field of the many
nuclei and other electrons that make up a molecule. The complexity of MOs can be dealt
with by constructing them from the AOs of the isolated atoms. The MOs are formed by
mixing the AOs based on the idea of interference described by the superposition of waves:
when waves come together in the same phase they reinforce one another, whereas waves
of opposite phase will tend to cancel each other out.
In this chapter we will find that only AOs of the same symmetry can mix by superpo-
sition to give MOs. To construct MOs, the symmetry of the AOs for the interacting atoms
are first established and then each matching set is used to produce SALCs which predict
the shapes of the MOs. This is simply an application of symmetry analysis to a basis of
AOs, and so we will always be able to construct the same number of MOs as there are AOs
in the basis.
Symmetry cannot tell us the relative energies of these orbitals, and so we also review
some ideas in chemical bonding and atomic electronegativity to allow the energetic order-
ing of MOs to be judged. We begin, in the next section, by outlining the idea of interference
and superposition of waves. Then we will discuss the link between MOs and the electron
density that is probed in experiment. The approach is to overview the general concepts of
MO theory using some quite complex-looking examples. Then, in later sections, a more
detailed analysis of chemical bonding and MO theory will be undertaken using simpler
cases in greater depth.
