# How to identify the most stable Lewis adduct of boron trichloride?

Which of the following is the most stable Lewis acid–base adduct?
(a) $$\ce{H2O\bond{->}BCl3}$$
(b) $$\ce{H2S\bond{->}BCl3}$$
(c) $$\ce{H3N\bond{->}BCl3}$$
(d) $$\ce{H3P\bond{->}BCl3}$$

This is a question from KVPY 2016 examination. In my first attempt, I focused on the positive charge that is going to appear on the donating atom. So the less electronegative the donor, the better it is at donating its lone pair to the vacant orbital on boron. However, the answer key states that the answer is option (c).

I am unable to rationalize why (c) is the most stable adduct.

• Since this is a Lewis acid-base interaction, the simplest way to go about it would have been to see the strongest Lewis base for this acid. As we move down a group, basicity decreases and as we move across a period,basicity again decreases. So ammonia is the strongest base here, and hence, answer would be (c) – Yusuf Hasan Dec 8 '18 at 2:50

Perhaps, what is expected of the question is for the student to use Pearson's Hard Soft Acid Base (HSAB) theory, which essentially states that hard acids have a preference to bind to hard bases and soft acids have a preference to bind to soft ligands. $$\ce {BCl3}$$ can be seen as a hard acid and would thus preferably interact with hard bases. Wikipedia classifies boranes as hard acids. Water and ammonia are much harder bases than phosphine and hydrogen sulfide since $$\ce {O}$$ and $$\ce {N}$$ are markedly more electronegative and possess higher electron density. Thus, we can likely expect either water or ammonia to be the correct option. Next, we need to compare between the two hard bases. Since they are both relatively hard, we need to compare now a different parameter, that is, their ability to donate the electron pair. In this aspect, nitrogen is clearly superior to oxygen due to the former's lower electronegativity. Thus, $$\ce {NH3}$$ is the correct option.