5.3.1.1: Molecular Orbitals
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There are several cases where our more elementary models of bonding (like Lewis Theory and Valence Bond Theory) fail to predict the actual molecular properties and reactivity. A classic example is the case of \(\ce{O_2}\) and its magnetic properties. At very cold temperatures, \(\ce{O_2}\) is attracted to a magnetic field, and thus it must be paramagnetic (unpaired electrons give rise to magnetism). Watch the video below !
The magnetic properties of \(\ce{O_2}\) are easily rationalized by its molecular orbital diagram . A molecular orbital diagram is a diagram that shows the relative energies and identities of each molecular orbital in a molecule. Figure \(\PageIndex{1}\) shows a simplified and generic molecular orbital diagram for a second-row homonuclear diatomic molecule. The diagram is simplified in that it assumes that interactions are limited to degenerate orbitals from two atoms ( see next section ).
There are some things you should note as you inspect Figure \(\PageIndex{1}\) (and keep these in mind as you draw your own diagrams!). First, notice that there are the same number of molecular orbitals as there are atomic orbitals . Second, notice that each orbital in the diagram is rigorously labeled using labels (\(\sigma\) and \(\pi\)) that include the subscripts \(u\) and \(g\). These labels and subscripts indicate the symmetry of the orbitals. The \(\sigma\) symbol indicates that the orbital is symmetric with respect to the internuclear axis, while the \(\pi\) label indicates that there is one node along that axis. The \(g\) and \(u\) stand for gerade and ungerade , the German words for even and uneven , respectively. The subscript \(g\) is given to orbitals that are even , or symmetric , with respect to an inversion center. The subscript \(u\) is given to orbitals that are uneven , or antisymetric , with respect to an inversion center. The pictures of calculated molecular orbitals are shown in Figure \(\PageIndex{1}\) to illustrate the symmetry of each orbital.
Another important thing to notice is that the diagram in Figure \(\PageIndex{1}\) lacks electrons (because it is generic for any second-row diatomic molecule). If this were a complete molecular orbital diagram it would include the electrons for each atom and for the molecule . Electrons in molecular orbitals are filled in the same way an atomic orbital diagram would be filled, where electrons occupy lower energy orbitals before higher energy orbitals, and electrons occupy empty degenerate orbitals before pairing. A complete molecular orbital diagram would show whether the molecule is diamagnetic or paramagnetic. It can also be used to calculate the bond order of the molecule (the number of bonds between atoms) using the formula below:
\[\text{Bond order } =\frac{1}{2}\left[\left(\begin{array}{c}\text { number of electrons } \\ \text { in bonding orbitals }\end{array}\right)-\left(\begin{array}{c}\text { number of electrons } \\ \text { in antibonding orbitals }\end{array}\right)\right] \nonumber \]
In general, non-valence (core) electrons can be ignored because they contribute nothing to the bond order. In fact, many molecular orbital diagrams will ignore the core orbitals, as they are insignificant for bonding interactions and reactivity.
Now, to see how this molecular orbital diagram can explain the magnetic behaviour of \(\ce{O2}\), complete the example below.
Example \(\PageIndex{1}\)
Let's change Figure \(\PageIndex{1}\) to make it specific for \(\ce{O2}\). Re-draw the MO diagram (no need to draw the shapes of orbitals). Fill in the correct number of electrons for each oxygen atom on either side of the diagram. Then, fill in the total molecular electrons in the center. Calculate the bond order and determine whether it is diamagnetic or paramagnetic.
Solution
The diagram in Figure \(\PageIndex{1}\) includes core orbitals (the 1s) and valence electrons (2s, 2p). Therefore, we will consider all the electrons in an oxygen atom and a dioxygen molecule. An oxygen atom has eight total electrons. So we fill eight electrons into the atomic orbitals for the oxygen atom on the right, and eight electrons into the atomic orbitals for oxygen on the left. The total number of electrons for the molecule is sixteen, so fill in 16 electrons into the molecular orbitals, being sure to apply Hund's rule and the Aufbau principle. The result is a diagram that looks like the one drawn below in Figure \(\PageIndex{2}\).
The bond order is calculated using the molecular orbitals (we can ignore atomic orbitals). There are 10 electrons in binding orbitals and 6 electrons in antibonding orbitals). This gives a bond order of \(\frac{1}{2}(10-6)=2\). This bond order is consistent with valence bond theory!
This diagram indicates that dioxygen is paramagnetic; it has two unpaired electrons in the \(\pi^*\) orbitals. This paramagnetic electron configuration explains why dioxygen is attacted to magnetic fields!
Exercise \(\PageIndex{1}\)
Draw the molecular orbital diagram for \(\ce{F2}\); be sure to label your orbitals with the appropriate symmetry and count your orbitals to make sure that the total number of atomic orbitals and molecular orbitals is the same. As a shortcut, include only the valence orbitals and electrons. What is the bond order? Is the molecule diamagnetic or paramagnetic?
- Answer
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The valence orbitals on an F atom are 2s and 2p. And, there are seven valence electrons in F. This gives fourteen total valence electrons. The MO diagram of the valence molecular orbitals can be constructed by combining the valence 2s and valence 2p orbitals from each F atom. The bond order is 1 and the molecule is diamagnetic.