Chemistry Is Like Sex: Coupling Illustrated

G.N. Lewis (I wrote about him back here) was from Berkeley and thus a bit more liberal when defining acids and bases: He gave more general definitions of them than Brønsted and Lowry did. And while more open-minded, Lewis was a bit of a chauvinist when he argued that a base's precious electrons helped "complete the octet" of an acid when they coupled.

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:

When NH₃ and BH₃ prepare to bond, a natural question is where should NH₃ put its lone pair?  BH₃ has what's called a "virtual orbital" (there's nothing virtuous about it).  A virtual orbital is just an empty electron orbital. Another name is a LUMO. Empty orbitals have metes and bounds, despite there being nothing there there. Here's a lurid depiction of borane's virtual orbital:

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.