Consider the coupling of a simple base, ammonia, with a simple Lewis acid, borane. If you're a jaded chemist who has "seen it all" you might consider just skipping to this link dealing with borane and ammonia making borazane as a hydrogen fuel energy source.
We already "know" what ammonia looks like here--but what about borane? I wrote a bit about boron the element back here. Turns out that the word "boron" is etymologically linked to the Arabic tongue as well, via the word borax.
Borane, BH3, is a natural fit for ammonia's lone pair. Consider its structure:
BH3 looks a bit like NH3 but completely lacks a lone pair. BH3 has only six surrounding electrons instead of eight and so is electronically unfulfilled. In a sense, it has a big hole in its middle. In the absence of an available lone pair, BH3 readily dimerizes in a head-to-tail fashion with another sister BH3 molecule to form B2H6. Here's an illustration of two BH3's getting it on together:
It's a bit hard to see in the depiction above but all three of borane's tripodal hydrogen limbs are squished flat into a planar configuration between the two swollen globes. The red and blue empty lobes are equivalent in the eyes of ammonia's incoming lone pair: Borane's empty orbital can be approached from above or below. As the ammonia approaches one side of borane, one empty lobe enlarges to accept the lone pair while the other shrinks. Also, borane's little hydrogen limbs fold back away from the incoming lone pair to accommodate the fit. The final coupling product looks like this:
BH3, with the help of ammonia's lone pair, now has an octet of electrons.