Question

The most stable Lewis acid-base adduct among the following is

• A. $H_2O \to BCl_3$
• B. $H_2S \to BCl_3$
• C. $H_3N \to BCl_3$
• D. $H_3P \to BCl_3$

Answer 1

Answer: (C)

A complex formed by the dative bond between Lewis acid and Lewis base is called Lewis acid-base adduct. In all the given cases Lewis acid, i.e. $BCl_3$ is same. So, stability depends on Lewis base.
Among the given options $H_{3} \to BCl_{3}$ form stable Lewis base adduct, because $N$ of $NH_{3}$ contains a lone pair and is less electronegative, thus it can donate its lone pair into empty orbital of $BCl_{3}$ (Lewis acid) to form effective dative bond $(p\pi-p\pi).$
In option $(a),$ O is highly electronegative, so it will not donate its lone pair to $BCl_3.$
In option $(b)$ and $(d),$ the formed dative bond $d\pi-p\pi$ is less effective than $p\pi-p\pi$ in case of $NH_3 \to BCl_3$.