MO5. Pi Bonding With p Orbitals
Earlier, we saw that p orbitals that lie along the same axis can interact to form bonds.
Parallel, but not collinear, p orbitals can also interact with each other. They would approach each other side by side, above and below the bond axis between the two atoms. They can be close enough to each other to overlap, although they do not overlap as strongly as orbitals lying along the bond axis. They can make an in-phase combination, as shown below.
They could also make an out-of-phase combination, as shown below.
- parallel p orbitals can overlap to produce bonding and antibonding combinations.
- the resulting orbitals contain nodes along the bond axis.
- the electron density is found above and below the bond axis.
- this is called a p (pi) bond.
The illustration above is for one set of p orbitals that are orthogonal to the bond axis. The second picture shows the result of the constructive (or destructive) interference. A similar picture could be shown for the other set of p orbitals.
In a main group diatomic species like dinitrogen, one p orbital lying along the bond axis can engage in s bonding. The two p orbitals orthoganol to the bond axis can engage in p bonding. There will be both bonding and antibonding combinations.
Just as the sigma-bonding orbitals display progressively shorter wavelengths along the bonding axis as they go to higher energy, so do the pi bonding orbitals. In other words, there are more nodes in the higher-energy orbitals than in the lower-energy ones.
An important consequence of the spatial distribution or "shape" of a p orbital is that it is not symmetric with respect to the bond axis. A s orbital is not affected when the atom at one end of the bond is rotated with respect to the other. A p orbital is affected by rotation. If one atom turns with respect to the other, the p orbital would have to stretch to maintain the connection. The orbitals would not be able to overlap, so the connection between the atoms would be lost.
Problem MO5.1.
When the 1s orbitals of two
hydrogen atoms combine to form a hydrogen molecule, which molecular orbitals
are formed?
Draw a picture.
Problem MO5.2.
In-phase combinations of
atomic orbitals give ______________ orbitals.
Draw a picture.
Problem MO5.3.
Out-of-phase combinations of
atomic orbitals give ______________ orbitals.
Draw a picture.
Problem MO5.4.
The combinations of ______________ atomic orbitals leads to σ orbitals.
Draw pictures.
Problem MO5.5.
The combinations of ______________ atomic orbitals leads to π orbitals.
Draw pictures.
Problem MO5.6.
Which molecular orbital is
typically the highest in energy?
a. p
b. σ
c. π*
d. π
e. σ*
Problem MO5.7.
Why can 1s orbital not
interact with a 2s orbital?
Hint: Why is a Li2O bond
stronger than a K2O bond?
Problem MO5.8.
Add a few words to explain the ideas conveyed in these drawings.
