pBlock ElementsII - Result Question 89
85. Account for the following. Write the answers in four or five sentences only.
(i) The experimentally determined $N-F$ bond lengths in $NF _3$ is greater than the sum of the single bond covalent radii of $N$ and $F$.
(ii) $Mg _3 N _2$ when reacted with water gives of $NH _3$ but $HCl$ is not obtained from $MgCl _2$ on reaction with water at room temperature.
(iii) $\left(SiH _3\right) _3 N$ is a weaker base than $\left(CH _3\right) _3 N$.
$(1995,2 M \times 3=6 M)$
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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 $NF _3, N$ and $F$ repel each other stretching the $N-F$ bond. Hence, in $NF _3$, $N-F$ bond lengths are greater than the sum of their single bond covalent radii.
(ii) $Mg _3 N _2+6 H _2 O \longrightarrow 3 Mg(OH) _2+2 NH _3$
$MgCl _2$ is a salt of strong acid $HCl$ and strong base $Mg(OH) _2$ and therefore, not hydrolysed in aqueous solution.
(iii) In $\left(SiH _3\right) _3 N$, 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.
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