# Lack of colour in Nitric Oxide

Usually compounds' colours are explained by the presence of unpaired electrons which is free to excite and de-excite giving the compound a charectistic colour. NO has atleast one unpaired electron but even then it is colourless. Can anyone tell me what the reason is?

You need more than an unpaired electron to create color in a substance. You need two places for it to be, with a given energy difference in the visible color range. Looking at the molecular orbital diagram of NO we see that no such organisation is possible (within reasons). The only choice is to go from one $\pi^*$ to the other, but those are at the same energy level. There is no way for this diatomic molecule to deform in any meaningful way so as to create a difference in energy levels between them. Metal complexes, especially iron- and cobalt-based ones, can do this, but not with the $\pi^*$ in any case. There is only $\sigma^*_2$ left, but the distance from $\pi^*$ to $\sigma^*_2$ is in huge, larger than visible spectrum (an event that knocks an electron up there is likely to unravel the entire molecule).