why NH3 & H2O are both strong & weak ligands? in both these, pairing will occur or not?

NH3 is moderately strong field ligand as here the donor atom is N which considerably electronegative.
In case of H2O, it is weaker ligand as compared to NH3 as the donor atom is Oxygen which is more electronegative than Nitrogen.
So as the electronegativity of central atom decrease, the ligand becomes stronger.
so in NH3, pairing will occur and in H2O pairing will not occur.

NH3 is a strong sigma doner than H2O which is a π doner. In NH3 donation to metal is through sigma HOMO making bond strong where as in H2O donation occurs From π HOMO which is a weak bond. This can be the possible reason as to why NH3 is strong field ligand thanH2O. Electonegativity cannot be the reason for this order as Cl with low electonegativity compared to N and O should have been a strong ligand which is never the case in accordance to spectrochemical series