Molecular Orbital Diagram of Polyatomic CO2 Molecules - Chemical Bonding & Molecular Structures

Описание

Carbon dioxide (CO2), molecule is triatomic and linear like Beryllium di hydride (BeH2) However, unlike hydrogen as peripheral atoms in BeH2, there are oxygen atoms in CO2, which have p orbitals. Hence there are more molecular orbitals in CO2 molecule than in BeH2 molecule. Electronic configuration of C and O atoms:
C (6) 1s2 2s2 2p2
O (8) 1s2 2s2 2p4
In CO2 molecule, carbon is central atom and two oxygen atoms are the peripheral atoms. So first orbitals of peripheral atoms I.e oxygen atoms will overlap to form group orbitals, and then these group orbitals will overlap with atomic orbitals of carbon atom. Now we will see the shapes of the group orbitals and try to match them with the shape of atomic orbitals of carbon atom simultaneously.
First of all 2s orbitals of oxygen atoms are not involved in bonding. So lets take 2pz orbitals of first and second oxygen atoms, they are aligned in the direction of axis of overlap. So they can be combined with the similar signs on each lobes to form group orbital psi 1. If we try to insert the carbon bot in between two oxygen atoms, the central lobe of group orbital can easily overlap with the 2s orbital of carbon atom. This type of overlap will produce sigma s and sigma star s orbitals if sign of these orbitals are opposite. If the sign of 2pz lobes of oxygen are different then subtraction takes place and psi 2 group orbital is formed, this is having matching symmetry with 2pz orbital of carbon atom. So they will give rise to two sigma bonds as sigma z and sigma star z depending on the sign of lobes.
Similarly 2px orbitals of oxygen combines to give psi 3 group orbital which have matching symmetry with the 2px orbital of carbon atom. So this will also give rise to two bonds, but in this case as the overlap lobes are at the perpendicular position of the overlap axis, instead of sigma bond, a pi bond is formed. They can be denoted by pi z and pi star z molecular orbitals. In case if signs of lobes of 2px orbitals of oxygen are different subtraction takes place to form group orbital pis 4 As you can see this group orbital have positive and negative at alternate ends, so there is no matching orbital in carbon atom. Thus this group orbital remains as non bonding molecular orbital. Similar to px orbital there is py orbital at an angle of 90 degree from the x and z direction, so they also combine to give group orbital psi 5 and this group orbital matches with the 2py orbital of carbon atom. So this will also make a pi bond as the overlap is perpendicular to the bond axis. Now if the signs of the lobes are opposite then susbtraction take place producing group orbital psi 6 this is not having any matching symmetry from the carbon atomic orbitals and remains as non bonding molecular orbital.
Now lets try to draw all these orbitals formed in the form of molecular orbital diagram. So again on the right hand side we have presented atomic orbitals of oxygen atoms, when they combine the produce 6 group orbitals. Now this group orbitals will overlap with atomic orbitals of carbon which is shown on the left hand side.
So first of all psi 1 overlaps with 2s orbital producing sign s and sigma star s molecular orbitals. Secondly psi 2 overlaps with 2pz orbitals of carbon, as this is axial overlap the bond formed will be sigma bond and they are sigma z and sigma star z.
Then psi 3 and psi 5 will overlap with px and py orbitals, as both the orbitals are at perpendicular to z axis, the overlap is lateral to the overlap axis, thus it forms pi bonds, So pi x and pi y bonding orbitals and pi star x and pi star y antibonding molecular orbitals are formed.
And lastly psi 4 and psi 6 remains as non bonding as there is no matching symmetry in carbon atom.
Now total 12 electrons four electrons of carbon and eight electrons of two oxygen atoms can be accommodated in these molecular orbitals from the lowest energy molecular orbitals.
Therefore from the M.O. diagram, the electronic configuration of CO2 molecule can be represented as (?s)2, (?z)2, (?x)2, (?y)2
As there is no unpaired electron in the molecular orbitals, CO2 is a diamagnetic molecule.

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