[ultima]

This may seem like a vague question, but I am trying to find out if anyone has details on when oxygen can act as a electron sink in reactions.

We are all familiar with this in carbonyl reactions, whereby a nucleophile can attack the carbonyl carbon, which (sometimes temporarily) pushes the electrons up to the oxygen, as the negative charge is favoured on the very electronegative atom.

However - Is this scenario the same with all types of oxygen bearing atoms?
For example, Phosphorus-Oxygen double bonds, Sulfur-Oxygen bonds and so on...

If I'm not describing myself very well, see the attached image.
Ive used very generalised examples - Nu attacking to substitute a chlorine.

Any information will be greatly appreciated.

[MethylNeptuneAcetate]

Well, it depends on what the LUMO is in those cases, whether it's the π* SO (or PO in the bottom example) or the σ* SCl (PCl). Steric hindrance (of both the attack of the nucleophile and the intermediate in the π* case) may also play a factor.

I'm not quite sure which is the LUMO in these cases (except in the top case where it's clearly π* CO, seeing as we know how that reaction progresses).

[ultima]

Ah okay. That does make simple sense.
So whatever orbital has the lowest energy will determine where the electrons initially attack.
I would guess that most of these kinds of molecules have lower energy S=O PI* orbitals (or P=O), and so electrons are going up to the oxygen.