[Vrig]

Does anybody know which pair of electrons are HOMO? Is it the non-bonding lone-pair or the π-orbital?
Keep in mind that the carbon and oxygen are both sp² hybridized and that the π-bond is due to p_z (if p_x and p_y are part of sp²) orbital overlap and hybridization lowers the orbital energy and oxygen is more electronegative.

[Dan]

Here is an example I found by googleing: carbonyl homo.