$\ce{NO}$ can't dimerize and that is explained because the free electron is in an antibonding orbital. The same is true for $\ce{NO2}$, but it can dimerize, how?
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Having an electron in an antibonding orbital means that the electron exists in a high energy orbital. If anything, this means the electron will be "motivated" to react and form a bond, particularly if bond formation lowers the energy of the electron. An electron in an antibonding orbital can form a bonding (as opposed to antibonding) bond with positive overlap.
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2$\begingroup$ Good question! Remember that NO only dimerizes at low temperatures. Apparently molecular oxygen does similar things. Since molecular oxygen has 2 unpaired electrons, maybe we would expect a polymer or a cyclic oligomer? It turns out that one form of solid oxygen actually exists as $\ce{O_8}$. Read about "red oxygen" or $\ce{O_8}$ in this Wikipedia article en.wikipedia.org/wiki/Solid_oxygen#Red_oxygen Here's a link to an article where $\ce{O_4}$ is reported onlinelibrary.wiley.com/doi/10.1002/… $\endgroup$– ronCommented Aug 16, 2014 at 13:49
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Are you sure that nitric oxide can't dimerize? It has a rather high bond order - 2.5 - and we can see that from molecular orbital theory. Therefore even if its free electron is in an antibonding orbital, that wouldn't decrease the bond order to 0 or below.
The nitric oxide dimer has actually been extensively studied too.
answered Aug 15, 2014 at 21:01