AB2 molecules like OF2 and O3

Further examples of molecules of point group C2v are Oxygendiflouride OF2 and the Sulfurdiflouride SF2. The number of valence electrons is the same for both molecules. For a start, we do not introduce the complete number of electrons of the flourine atoms. Instead, we concentrate on those electrons appropriate for bonds of rotational symmetry. These electrons are found in the first and the second row of the table below.

atomic orbitalable to formways of notation
pxpπ-bondpπ-AO or π-AO
pypπ-bondpπ-AO or π-AO

Regarding only the pair of electrons in the σ-AO and the single electron in an pσ-AO of each flourine atom, the molecules OF2 and SF2 appear analogous to the previously dicussed water molecule. With this constrictions, we recieve the following series of molecular orbitals. The background color in the table signifies the role for molecular bonding: Blue for bonding, green for non-bonding and red for anti-bonding.

1a1 2a1 3a1 4a1 5a1
1b2 2b2 3b2

These eight orbitals harbour twelve electrons (6 of oxygen / sulfur and 3 from each flourine atom). Three bonding and two non-bonding orbitals accept a maximal number of ten electrons. For two electrons, only an antibonding orbital remains. Among the two antibonding orbitals, the 3a1 is lower in energy and thus filled. With respect to molecular energy, 2a1 and 3a1 compensate each other. According to this approach, the formation and stability of the molecules OF2 and SF2 is based on two bonding molecular orbitals.

Now we want to introduce the electrons in π-AOs (or, more exactly, the electrons of pπ-orbitals) into our consideration. We then have the following series of molecular orbitals:

1a1 2a1 3a1 4a1 5a1
1b1 2b1
1b2 2b2 3b2 4b2

A number of five molecular orbitals are bonding and two are non-bonding. Together, they accept 14 electrons, but now, a total of 20 valence electrons (7 from both flourine atoms) are to be distributed. Therefore three antibonding MOs, namely 4a1, 2b1 and 3b2 must harbour the six remaining electrons. We would expect the effects of five fully occupied bonding and three fully occupied antibonding orbitals to compensate each other partially. In fact, the molecules appear to be bound by only two molecular orbitals. Thus, the newly introduced electrons of flourine's &pi-AOs seem to be distributed equally among bonding and antibonding molecular orbital, having no overall effect.

We may use the same scheme of molecular orbitals to discuss the bonding within a molecule of ozone O3 or sulfurdioxide SO2. Flourine and oxygen differ by one valence electron. We therefore have 18 instead of 20 electrons and one more antibonding orbital remains empty. This would explain why ozone and sulfurdioxide are more stable than their flourine analogons.

Similar to diatomics, in molecules of the class ABn, electrons in &sigma-; pσ- and pπ-states form bonds classified as σ- or π-MOs. Note that there is no true separation because the σ- and π-electrons and there are just a few symmetry groups where molecular orbitals are either formed from σ- or from π-orbitals. A table presents an overview. For example, such a separation

Let us get back to the example of ozone O3 and sulfurdioxide SO2. If we have one fully occupied bonding orbital of type 1b1 and one empty antibonding orbital of symmetry species 2b1, we would conclude that, among the remaining bonding orbitals, there is one of type π. A quantitative treatment shows that the other two molecular orbitals are mainly of σ-type. Therefore, in ozone and in sulfurdioxide are described as formed by two σ- and one π-bond.

In Oxygendifluoride OF2 and Sulfurdiflouride SF2, besides the 1b1 the antibonding π-orbital 2b1 is doubly occupied. Therefore there are two bonds of type σ and none of type π. The first simplified approach to this molecule which ignored the π-electrons led to the same picture.

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