Question

Point out incorrect stability order.

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

Answer 1

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

Increasing order of stability
(a) ${\left[Cu{\left(N{H}_3\right)}_4\right]}^{2+}<{\left[Cu{\left(en\right)}_2\right]}^{2+}<{\left[Cu\left(trien\right)\right]}^{2+}$
Their formation of entropy increases in the same order. Ligand denticity is increased.
(b) ${\left[Fe{\left(H_{2}O\right)}_6\right]}^{3+}<{\left[Fe{\left(N{O}_2\right)}_6\right]}^{3-}<{\left[Fe{\left(N{H}_3\right)}_6\right]}^{3+}$
$\because$ $N{H}_3$ is weaker ligand than $N{O}_2^{-}$.
$\therefore$ The correct stability order is
${\left[Fe{\left(H_{2}O\right)}_6\right]}^{3+}<{\left[Fe{\left(N{H}_3\right)}_6\right]}^{3+}<{\left[Fe{\left(N{O}_2\right)}_6\right]}^{3-}$
(c) ${\left[Co{\left(H_{2}O\right)}_6\right]}^{3+}<{\left[Rh{\left(H_{2}O\right)}_6\right]}^{3+}<{\left[Ir{\left(H_{2}O\right)}_6\right]}^{3+}$
$Z_{eff}$ value increases from ${Co}^{3+}$ to ${Ir}^{3+}$.
(d) ${\left[Cr{\left(N{H}_3\right)}_6\right]}^{+}<{\left[Cr{\left(N{H}_3\right)}_6\right]}^{2+}<{\left[Cr{\left(N{H}_3\right)}_6\right]}^{3+}$
O.S of $Cr$ atom increases from $+1$ to $+3$.