Question
Consider the metal complexes $\left\lbrack Ni(en)_{3} \right\rbrack^{2 +}(A)$, $\left\lbrack {NiCl}_{4} \right\rbrack^{2 -}(B)$ and $\left\lbrack Ni\left( {NH}_{3} \right)_{6} \right\rbrack^{2 +}(C)$. Choose the CORRECT option by considering the number of unpaired electron present in (A), (B) and (C) respectively and the order of frequency of absorption.
Options
Solution
Sol. [Key Concept: All three Ni$2 +$ ($d^{8}$) complexes have 2 unpaired electrons; the absorption frequency follows $\Delta_{o}$, i.e. ligand strength en $>$ NH$3 >$ Cl$-$.]
(A) ${Ni}^{2 +} \Rightarrow 3{\text{ }d}^{8},SFL,t_{2\text{ }g}^{2,2,2}e_{g}^{1,1} \Rightarrow$ unpaired $e^{-} = 2$
(B) ${Ni}^{2 +} \Rightarrow 3{\text{ }d}^{8}$, WFL, tetrahedral, $e^{2,2}t_{2}^{2,1,1}$
⇒ unpaired $e^{-} = 2$
(C) ${Ni}^{2 +} \Rightarrow 3{\text{ }d}^{8}$, SFL , $t_{2\text{ }g}^{2,2,2}e_{g}^{1,1} \Rightarrow$ unpaired $e^{-} = 2$
As $\Delta_{o} > \Delta_{t}$ and Ligand strength : en $> {NH}_{3} > {Cl}^{-}$
Strength of ligand increases, $\Delta$ increases, hence more is the frequency required of absorption.
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