pBlock ElementsII 1 Question 89
85. Account for the following. Write the answers in four or five sentences only.
(i) The experimentally determined $\mathrm{N}-\mathrm{F}$ bond lengths in $\mathrm{NF}_{3}$ is greater than the sum of the single bond covalent radii of $\mathrm{N}$ and $\mathrm{F}$.
(ii) $\mathrm{Mg}{3} \mathrm{~N}{2}$ when reacted with water gives of $\mathrm{NH}{3}$ but $\mathrm{HCl}$ is not obtained from $\mathrm{MgCl}{2}$ on reaction with water at room temperature.
(iii) $\left(\mathrm{SiH}{3}\right){3} \mathrm{~N}$ is a weaker base than $\left(\mathrm{CH}{3}\right){3} \mathrm{~N}$.
$(1995,2 \mathrm{M} \times 3=6 \mathrm{M})$
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Solution:
- (i) The size of both nitrogen and fluorine are very small as well as they have very high electron density. Thus in $\mathrm{NF}{3}, \mathrm{~N}$ and $\mathrm{F}$ repel each other stretching the $\mathrm{N}-\mathrm{F}$ bond. Hence, in $\mathrm{NF}{3}$, $\mathrm{N}-\mathrm{F}$ bond lengths are greater than the sum of their single bond covalent radii.
(ii) $\mathrm{Mg}{3} \mathrm{~N}{2}+6 \mathrm{H}{2} \mathrm{O} \longrightarrow 3 \mathrm{Mg}(\mathrm{OH}){2}+2 \mathrm{NH}_{3}$
$\mathrm{MgCl}{2}$ is a salt of strong acid $\mathrm{HCl}$ and strong base $\mathrm{Mg}(\mathrm{OH}){2}$ and therefore, not hydrolysed in aqueous solution.
(iii) In $\left(\mathrm{SiH}{3}\right){3} \mathrm{~N}$, the lone pair of nitrogen is involved in $p \pi-d \pi$ bonding, less available on nitrogen for donation to a Lewis acid, a weaker Lewis base
Carbon does not have any vacant $d$-orbitals, no such $p \pi-d \pi$ bonding occur in trimethyl amine, lone pair of nitrogen is available for donation to Lewis acid, hence a stronger Lewis base.
