[orgohell]

Hey, I'm having trouble understanding why the electron configuration is different between C₂ and O₂, as in why is the 2sσ lower in energy then 2pi in one, but is the opposite in the other? How would I know how to set up the configuration if it was say CO?

[cheese (MSW)]

Zeff on the 2s electron increases more rapidly than Zeff on the 2p electron because the 2s penetrates the core 1s2 AOs more effectively than the 2p electrons and experiences a larger nuclear charge.  As a consequence the 2s/2p energy gap gets larger going from Li → F.  It is estimated that the 2s-2p energy gap is ~200 kJ mol-1 at Li and increases to ~2500 kJ mol-1 at F.
Because of the large sp energy gap the 2s AO overlap on atom A with the 2pz AO on atom B is not significant and can be ignored for O2 (and its various ions) and F2. Thus we have the simple MO diagram with no sp mixing, e.g., for O2:
O2 = 12 valence e⁻ = σs(2e⁻) σs*(2e⁻) σp(2e⁻) πp(4e⁻) πp*(2e⁻) σp*(0)   
For elements of the first row that are to the left of oxygen in the periodic table the 2s-2p energy gap is smaller.  For this reason the overlap of the 2s AO on one atom with the 2p orbital on the other atom becomes significant. This is called s-p mixing.  The effect of this interaction is typical for AO overlaps:  the two lower energy σ MOs are further lowered in energy and the two higher energy σ MOs are raised in energy.  Of importance is that the third σ3 MO (originally σp) is raised above the π1 MO.  Because the σ MOs have both s and p character and the subscripts are changed to numbers (σ1 – σ4).  For example, the MO diagram for CO (and NO) is:
CO: 10 valence e-  σ1(2e-) σ2 (2e-) π1(4e-) σ3(2e-)      π2*(0e-) σ4*(0e-)
It is crucial in the understanding of bonding of CO to TM (as Ni(CO)4) that the HOMO is a σ MO.
The Schrödinger Wave Equation explains it all.