OK, so we all love the homonuclear diatomics, and how the 2p sigma is *below* the 2p pi for O2 and F2, almost equal at N2, and above 2p pi for C2, B2, etc.
CO looks a lot like N2, but it's asymmetric...the 2p sigma is the HOMO. Disturbingly, the gap seems to be *larger* than in N2...why is that?!? Being isoelectronic, it seems like it should be about the same, but there should be *more* contribution from the oxygen orbitals in the *bonding* orbitals since O is more electronegative than C.
It gets weirder for NO...while I can't find a photoelectron spectrum, all the written diagrams I can find place the sigma well above the pi. Weirdly, a low-level Spartan (TM) calc shows the sigma *between* the pi orbitals (yes, I understand they unpaired electron in the pi* breaks the symmetry). It seems like if anything, NO should be closer to O2 than N2, and N2 has nearly equal energies....
What gives here? What do I not understand about sp mixing that is causing me to get this so wrong?