In which of the following pairs of molecules/ions, the central atoms have $\mathrm{sp}^2$ hybridisation?
In which of the following pairs of molecules/ions, the central atoms have $\mathrm{sp}^2$ hybridisation?
$\mathrm{NO}_2^{-}$and $\mathrm{NH}_3$
$\mathrm{BF}_3$ and $\mathrm{NO}_2^{-}$
$\mathrm{NH}_2^{-}$and $\mathrm{H}_2 \mathrm{O}$
$\mathrm{BF}_3$ and $\mathrm{NH}_2^{-}$
Solution
Key Idea For $\mathrm{sp}^2$ hybridisation, there must be $3 \sigma$-bonds or $2 \sigma$-bonds along with a lone pair of electrons.
(i) $\mathrm{NO}_2^{-} \Rightarrow 2 \sigma+1 \mathrm{lp}=3$, ie, $\mathrm{sp}^2$ hybridisation
(ii) $\mathrm{NH}_3 \Rightarrow 3 \sigma+1 \mathrm{lp}=4$, ie, $\mathrm{sp}^3$ hybridisation
(iii) $\mathrm{BF}_3 \Rightarrow 3 \sigma+0 \mathrm{lp}=3$, ie, $\mathrm{sp}^2$ hybridisation
(iv) $\mathrm{NH}_2^{-} \Rightarrow 2 \sigma+2 \mathrm{lp}=4$, ie, $\mathrm{sp}^3$ hybridisation
(v) $\mathrm{H}_2 \mathrm{O} \Rightarrow 2 \sigma+2 \mathrm{lp}=4$, ie, $\mathrm{sp}^3$ hybridisation
Thus, among the given pairs, only $\mathrm{BF}_3$ and $\mathrm{NO}_2^{-}$ have $\mathrm{sp}^2$ hybridisation.