is C2O4(2-) a weak ligand or strong ligand?

Oxalate is the conjugate base of oxalic acid, which is an excellent ligand for metal ions. It usually binds as a bidentate ligand forming a 5-membered MO₂C₂ ring. It uses its two oxygen atom for binding.
Example:-
Potassium ferrioxalate i.e.   K₃[Fe(C₂O₄)₃]