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​Q9. Which stability order is incorrect 

      (1) [ Fe(C2O4)3]3– > [Fe(H2O)6]3+                          ​  (2) [ Fe(EDTA)]2– > [Fe(en)3]3+

      ​(3) [ Ni(en)2]2+ > [Ni(dmg)2]                                              (4) [ Fe(CN)6]3– > [Fe(CN)6]4–

Dear student,
In case of stability of a complex compound, it depends on two factors.
1. Presence of strong field ligand.
2. Presence of chelating ligand
3.Oxidation state of central metal ion.

In option 1, Oxalate ion is a chelating ligand as it is a bidentate ligand hence its complex must be more stable, hence this is correct.
In option 2, in both the complex, the ligands are chelating and both are strong field ligand but in case of [Fe(EDTA)]-2, the oxidation of Fe is +2 whereas in [Fe(en)3]+3, it is +3, hence this is incorrect.
In option 3, en or ethylene diamine is a strong field ligand as compared to dmg or dimethyl glyoxime, hence this is correct.
In option 4, 1st complex has Fe with oxidation state +3 whereas 2nd one has oxidation state of Fe as +2. Hence this is correct.

Hence the answer is option (2).

Hope it helps

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