Classifying acid-base reaction between aluminium hydride and lithium hydride

Question

So I was dealing with a reaction the other day, which is also briefly written in the title:

$$\ce{AlH3(s) + LiH(aq) -> Li+(aq) + AlH4-(aq)}$$

The strange thing is:

• Arrhenius definition cannot explain the reaction, since there is no $\ce{OH-}$ in the reaction formula.

• Also Brønsted–Lowry definition cannot explain the reaction, because the stuff which $\ce{LiH}$ gives to $\ce{AlH3}$ is not a proton (hydrogenium, $\ce{H+}$) but a protide (hydride, $\ce{H-}$).

• And as far as I know, this is the exact definition of Lewis acid and base written in my chemistry textbook:

• Lewis acid: A substance which can receive unshared electron pair
• Lewis base: A substance which can give unshared electron pair

leading to a conclusion that the above reaction also cannot be explained through the Lewis definition. Below are my questions:

• Is there anything wrong in my thought?
• Is that really an acid-base reaction? If it is, what kind of acid-base reaction is it?

Answer 1

This is indeed a Lewis acid-base reaction.

The hydride ion has a lone pair which is, in general, not bound to anything. The aluminium atom has at least one unpopulated orbital (typically taken to be a $\mathrm p_z$ orbital in a planar $\ce{AlX3}$ environment). This makes hydride a Lewis base and aluminium a Lewis acid. The association of hydride to aluminium is thus a Lewis acid-base reaction.