electron counting for transition metal-oxo ligand compound

Using donor-pair method, how many electrons would an oxo ($O^{2-}$) ligand contribute? I cannot decide if it is 2 or 4.




Ok


So a very large ligand like ETDA are hexadentate


But


I thought of different metals


Different metals can accept different amount of pairs in 1 oxo ligands


For example MnO4-
Yes, it can hold total of 7pairs, and accept 4 electrons per oxo.


And then for W
Tungsten is very large, and can therefore accept a lot of electrons from one O2-, in fact, it can handle 3 pairs, or 6 electrons


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