CO in high concentration is fatal in less than 5 mins !! The worst part is that this is an odorless and colorless gas and so it becomes very difficult to detect its presence. This gas is a by-product of burning fuel in vehicles, grills, etc. The weight of CO is similar to that of air. So, this gas does not rise or sink but completely mixes in the air, making it more dangerous.
In this post, we shall study the CO molecule’s MO diagram at length. The carbon and oxygen are bonded by three bonds and we hope to deduce this structure by applying the MOT theory.
Carbon monoxide (CO) –
CO comprises of two atoms- carbon and oxygen. From the periodic table, we know that oxygen is farther to the right in the second period than carbon and so is more electronegative than carbon. Thus, the energy of AOs of oxygen will be lower in energy than that of carbon.
As seen in the figure above, the energies of 2s (of carbon atom), 2s, and 2p (of oxygen atom) are comparable. The 2s of oxygen is lower in energy(-32.4eV) as the electron in this orbital is tightly bound to the nucleus with a high charge. Thus while drawing the MO diagram, we have to show interactions between the 2s of C,2s of O, and 2p of O atoms. There are weak interactions between the 2s of O and other orbitals of C and O too. The MO diagram can be drawn as follows –
- In the above diagram, the atomic orbitals of carbon are shown in red color , atomic orbitals of oxygen are shown in blue color and the molecular orbitals of CO are shown in pink.
- Carbon has four valence electrons two in 2s and two in 2p orbital. Oxygen has six valence electrons – two in 2s and four in 2p orbitals.
- As seen in the figure, there are interactions between 2s and 2p orbitals and thus the MO diagram looks more complicated.
- The MO σ2s orbital is higher in energy than the 2s of oxygen atom , as it is formed by interactions from both 2s and 2p orbitals on both atoms.
- The bonding orbitals are closer to oxygen atom and the anti bonding ones closer to carbon.
- The 2s of carbon and 2s of oxygen atoms overlap as follows-
- The 2pz orbitals on both carbon and oxygen overlap to form σ2p and σ*2p orbitals as follows –
The carbon lobes are bigger as both 2s and 2p of carbon contribute in making these orbitals. Thus, these two orbitals are more on carbon than oxygen.The σ2p is called HOMO – Highest Occupied Molecular Orbital
The orbital σ*2p is called LUMO – Lowest Unoccupied Molecular Orbital.
HOMO means the MO with the highest energy, which is filled with electrons. As seen in the above MO diagram, the σ2p orbital is with the highest energy and is also occupied. All other orbitals above it do not have electrons in them. The σ*2p is the orbital with the lowest energy that is unoccupied by electrons. It is thus the LUMO. HOMO and LUMO are responsible for the reactions that the molecule undergoes. As both these orbitals are more on carbon, it is the carbon end, which is responsible for all reactions involving the CO molecule.
(We will use the HOMO – LUMO concept a lot in the sigmatropic reactions chapter, which we shall discuss in later posts.)
∴BO for CO = 1/2 (8 – 2) = 1/2(6) = 3.
Thus, we can see how MOT theory correctly predicts a triple bond between carbon and oxygen. This theory also shows us how a MO can have a contribution from more than two AOs. In complex molecules, it thus becomes very difficult to draw MO diagrams with precision. Computers are used to correctly predict such complexity.
In the next chapter, we will learn a new type of bond which forms the basis of an entire class of compounds.
Be a perpetual student of life and keep learning …
Image source –
References and Further Reading –