Question

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

• A. $H_{2}O\to BC{l}_3$
• B. $H_{2}S\to BC{l}_3$
• C. $H_{3}N\to BC{l}_3$
• D. $H_{3}P\to BC{l}_3$

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. $BC{l}_3$ is same. So, stability depends on Lewis base.
Among the given options $H_3\to BC{l}_3$ form stable Lewis base adduct, because $N$ of $N{H}_3$ contains a lone pair and is less electronegative, thus it can donate its lone pair into empty orbital of $BC{l}_3$ (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 $BC{l}_3.$
In option $(b)$ and $(d),$ the formed dative bond $d\pi -p\pi$ is less effective than $p\pi -p\pi$ in case of $N{H}_3\to BC{l}_3$.