Question

From the following, choose the incorrect order of stability:

• A. $\left[Cu\right(N{H}_3{)}_4{]}^{2+}<\left[Cu\right(en{)}_2{]}^{2+}<\left[Cu\right(trien){]}^{2+}$
• B. $\left[Fe\right(H_{2}O{)}_6{]}^{2+}<\left[Fe\right(H_2{O}_2{)}_6{]}^{3+}<\left[Co\right(N{H}_3{)}_6{]}^{3+}$
• C. $\left[Co\right(H_{2}O{)}_6{]}^{3+}<\left[Rh\right(H_{2}O{)}_6{]}^{2+}<\left[Ir\right(H_{2}O{)}_6{]}^{3+}$
• D. $\left[Fe\right(H_{2}O{)}_6{]}^{3+}<\left[Fe\right(N{O}_2{)}_6{]}^{3-}<\left[Fe\right(N{H}_3{)}_6{]}^{3+}$

Answer 1

The correct option is D $\left[Fe\right(H_{2}O{)}_6{]}^{3+}<\left[Fe\right(N{O}_2{)}_6{]}^{3-}<\left[Fe\right(N{H}_3{)}_6{]}^{3+}$

Stability of coordination complex depends on the number of rings in a chelate complex.
Thus, option (a) is correct.
$\left[Cu\right(N{H}_3{)}_4{]}^{2+}<\left[Cu\right(en{)}_2{]}^{2+}<\left[Cu\right(trien){]}^{2+}$

Strong field ligand will stabilise the complex more than weak field. $N{H}_3$ has strong field strength than $H_{2}O$.
Stability also depends on the oxidation state of the metal ion when it has same ligands and same coordination number.
Thus, order of stablility given in option (c) is correct.
$N{O}_2^{2-}$ has strong field strength than $N{H}_3$. Hence, stability order given in option (b) is incorrect.

For the same ligand and oxidation state, the crystal field splitting is proportional to the size of metal ion.
Thus, the order of CFSE is
$3d<4d<5d$
Hence, option (d) is also correct.
$\left[Co\right(H_{2}O{)}_6{]}^{3+}<\left[Rh\right(H_{2}O{)}_6{]}^{2+}<\left[Ir\right(H_{2}O{)}_6{]}^{3+}$