So far, we have taken care of 1s and 2s orbitals of C and 1s, 2s and one 2p, say 2p_z of O. The two other 2p orbitals of O, say 2p_x and 2p_y orbitals of C to give two bonding MOs (1 and 2) and two antibonding MOs ( 3and 4) which are higher in energy. The 2p_z orbital of C has no counterpart in O to bond with and hence it remains a non bonding orbital (6). This diagram, which shows how the MO energy levels relate to the energy levels of the AOs of the atom is called a correlation diagram. The electronic configuration of CO will then be 1² 2²3² 4²1 ²2²5². This takes care of all the 14 electrons in CO.

There are no electrons in the 6

, 3

and 4

levels. The highest occupied molecular orbital is 5

, which has a dominant contribution from the 2p_z orbitals of oxygen. In an MO, the AO whose energy level is closer to the MO energy level, contributes to a greater extent (> 50 %) to the MO. In this model where hybridization is not invoked, the participating AO "pairs" such as 2s₁ and 2p₂ and 2p₁ and 2p₂ are not very close in their energy values) and hence the binding resulting from such overlaps is not very strong.

Now consider the MO correlation diagram in fig 8.5 (b). There is hybridization in both the atoms between their 2s and 2p_zAOs. The hybrid orbitals in C are h₁ and h₂ and h₃ and h₄ are the hybrid orbitals in O. Note that the energies of these hybrid orbitals lie between the energy levels of the corresponding AOs.

What has happened as a consequence of hybridization is that the energy level h₁ (of C) has come very close (in value, i.e. in height in fig 8.4 (b), where height represents the energy of the orbital) to the energy of h₄ which is the hybrid in the oxygen atom. The hybrid orbitals h₁ and h₄ together form a strongly bonding MO, 4 and a highly antibonding MO, 6. The hybrid orbitals h₃ (of O) and h₂ (of C) remain non-bonding. The energies of the orbitals take on the same values as in the unhybridized case. Although the electronic configuration in 8.5 (b) remains the same in case of CO as in the unhybridized case (8.5(a)), there are two crucial differences between these. One is that the bond order in the unhybridized CO is 2 while it is 3 in hybridized CO.

The binding energy in CO is -256 kcal /mol while the binding energy in N₂ is -240 Kcalmol - 1. CO is one of the strongest diatomics and the strength can be only explained using hybridization. The higher binding in CO (relative to N₂) is due to electronegativity differences between C and O and the consequent differences in the MO energy levels.The second difference is that the highest occupied MO (HOMO) in the unhybridized case is dominantly an oxygen orbital (mainly oxygen 2p with some contribution from carbon 2s) whereas the HOMO in the hybrided CO is a nonbonding hybrid on C, which is mainly an atomic orbital. This lone pair of the outermost orbital of C is available for donating ("coordinate" bond) to electron deficient sites such as metal carbonyls. It is experimentally found that in metal carbonyls, CO binds to metals from the carbon end rather than the oxygen end (even though oxygen is more electronegative).