Statement A is correct. The oxalate ion (C2O42−) is a bidentate ligand and forms chelate rings with the central metal ion. Due to the chelate effect, [Fe(C2O4)3]3− is more stable than complexes formed by monodentate ligands like [Fe(OH)6]3− and [Fe(SCN)6]3−.
Statement B is incorrect. Ethylenediamine (en) is a bidentate ligand and forms chelate rings, whereas NH3 is a monodentate ligand. Therefore, [Cu(en)2]2+ is more stable than [Cu(NH3)4]2+ due to the chelate effect.
Statement C is correct. In K4[Fe(CN)6], the oxidation state of Fe is +2, which corresponds to a 3d6 configuration. Since CN− is a strong field ligand, it causes the pairing of electrons in the 3d orbitals, leaving two inner 3d orbitals empty. Thus, the hybridization is d2sp3.
Statement D is correct. The NO2− ligand is an ambidentate ligand, meaning it can coordinate to the metal center through either the nitrogen atom or the oxygen atom. Therefore, [Fe(NO2)3Cl3]3− can exhibit linkage isomerism.
Statement E is incorrect. Both NO2− and SCN− are ambidentate ligands. NO2− can bind through N or O, and SCN− can bind through S or N.
Therefore, the correct statements are A, C, and D.
Answer: A, C and D only