Paramagnetic character depend on the number of unpaired electron present in any complex. In [CoF6]3–, Co is having four unpaired electrons as F– does not cause pairing of electrons in d-orbitals, whereas in case of [Co(C2O4)3]3–, C2O4 causes pairing of unpaired electrons in d–orbitals and thus there is no unpaired electron, hence it is diamagnetic in nature.