Account for the following :
(i)The experimentally determined `N-F` bond length 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 `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`.

Explanation

The bond length is high due to repulsion of the bonded pair by both nitrogen and fluorine atoms. This is due to their smaller size and electron density.