Question

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) $Mg_3N_2$ when reacted with water gives of $NH_3$ but $HCl$ is not obtained from $MgCl_2$ on reaction with water at room temperature.
(iii) $(SiH_3)_3 N $ is a weaker base than $(CH_3 )_3 N.$

Answer 1

(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) $Mg_3N_2 + 6 H_2O \longrightarrow 3Mg(OH)_2 + 2NH_3$
$MgCl_2$ is a salt of strong acid $HCl$ and strong base $Mg(OH)_2$ and therefore, not hydrolysed in aqueous solution.
(iii) In $(SiH_3)_3 N$, 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.