Question

Select the incorrect order of stability :

• A. $\left[Ni\right(dmg{)}_2]>[Ni(en{)}_2{]}^{2+}$
• B. $\left[Fe\right(EDTA){]}^{2-}>[Fe\left(EDTA\right){]}^{-}$
• C. $\left[Co\right(Ox{)}_3{]}^{3-}>\left[Co\right(N{H}_3{)}_6{]}^{3+}$
• D. $\left[Co\right(N{H}_3{)}_6{]}^{3+}>\left[Co\right(H_{2}O{)}_6{]}^{3+}$

Answer 1

The correct option is B $\left[Fe\right(EDTA){]}^{2-}>[Fe\left(EDTA\right){]}^{-}$

(a). Stability of coordination complex depends on the number of rings in a chelate complex.
$\left[Ni\right(dmg{)}_2]$ complex for 4 rings in the complex whereas $\left[Ni\right(en{)}_2{]}^{2+}$ forms 2 rings.
Hence, $\left[Ni\right(dmg{)}_2]>[Ni(en{)}_2{]}^{2+}$

(b). The charge on EDTA is -4
Thus, oxidation state of $\left[Fe\right(EDTA){]}^{2-}$ is +2 and oxidation state of $\left[Fe\right(EDTA){]}^{-}$ is +3
For same metal ion, same ligand and fixed coordination number, the $CFSE\propto \text{oxidation state of metal}$
Metal with higher oxidation state will have higher $CFSE$ value.
Hence, $\left[Fe\right(EDTA){]}^{2-}<[Fe\left(EDTA\right){]}^{-}$
Thus, option (b) is incorrect.

(c). Oxalate is a chelating ligand hence, its complex is more stable.

(d). $N{H}_3$ is strong field ligand compare to $H_{2}O$. Hence option (d) is correct.