# Order of filling of molecular orbitals in heteronuclear diatomic molecules such as CO [duplicate]

Currently studying for possibly my last Chemistry exam in my life, can't understand why physics undergrads have to learn chemistry as high level as this, but oh well.

So I'm currently dealing with Molecular Orbital theory to predict the strength of bonds between various atoms and whether the resulting molecule will be paramagnetic or diamagnetic.

Now, I have discovered that for homonuclear diatomic molecules, there are two different molecular orbital filling orders, as in, for Li$_2$ all the way to N$_2$, the two $\pi_{2p}$ orbitals are filled before the $\sigma_{2p}$ orbital is filled.

From O$_2$ onwards, the $\sigma_{2p}$ orbital is less energetic than the two $\pi_{2p}$ orbitals, and thus the $\sigma_{2p}$ orbital is filled before.

For certain heteronuclear diatomic molecules, such as CN$^-$, the order of filling is the same as in N$_2$, and in molecules like OF$^+$, it appears to be the same as in O$_2$.

However, in "mixed" molecules such as CO or CO$^+$, how does one predict the order of filling of the molecular orbitals? It appears to be $\sigma_{2p}$ first, then the two $\pi_{2p}$ orbitals, following the fashion of O$_2$.

Is there a rule of thumb on how to predict which molecular orbital will be more energetic in molecules where the two atoms follow different orbital filling orders in their corresponding diatomic molecules, or is there not?

We are only asked to be able to do this for atoms lighter that Ne, it seems, so we only care about $2p$ orbitals.

• Good question. Just a point of clarification, however, you talk about 'orbital filling' - orbitals are filled bottom up, always. I think what you're trying to get at is the ordering of the orbitals themselves (i.e. does s-p mixing take place). – NotEvans. Feb 3 '18 at 19:31
• @NotEvans. Yes, that is the question. Pretty much which orbital is more energetic in which case. – ChemiCalChems Feb 3 '18 at 19:32