Suggest a structure for the mixed aluminum–boron hydride AlBH₆.

Video transcript

Hi, everyone. Welcome back. Here's our next problem, propose a plausible structure for BGAH six, a mixed hydride of boron and gallium. So let's think about how boron and gallium would interact with hydrogen. We can look up on our periodic table that gallium is in three A along with boron. So group three A elements with three valence electrons. So we can imagine they might interact with hydrogen in a similar way. Well, hopefully recall that boron has a very unusual way of interacting with hydrogen. If we look at bh three, if you tried to make this boron has just three valence electrons, and if three bonds with hydrogen, there's no lone pair and you've got this empty p orbital in boron. So this is an unstable arrangement instead, boron tends to f boron hydride tends to exist as this dimer B two H six, which of this rather unusual arrangement where you have two hydrogens as sort of a bridge between the two boron atoms. So you have boron and boron connected with two hydrogens and then each boron has two more hydrogens on the outside. There's our B two H six. Well, why does it exist this way we see right away, we should be struck by the fact that these hydrogens have two bonds each, which seems very wrong. Hydrogen just doesn't do that. And they can't be classic bonds because of course, we don't have enough electrons here. The structure is electron deficient because boron with three valence electrons. So three electrons times two give six electrons, hydrogen has one electron times six hydrogens equals six electrons. So this molecule has only 12 valence electrons and we have 12345678 bonds that would need 16 electrons. We don't have enough for the bonds if they're typical bonds on an a coal and structure. So boron there has four bonds. So we'd expect an sp three hybridization. So I'm just drawing some sp three orbitals coming out. We're gonna have to imagine it as a tetrahedral structure looks flat on my drawing and the outer bonds with the four outer hydrogens are just typical two electrons shared with two atoms. So we'd call that two center, two electron bonds. So that's the case for all four outer hydrogens. But we're gonna have to have something strange going on with our bridging hydrogens. So we know that each hydrogen has one electron and each boron can only contribute one electron to this arrangement since it's sharing its other two valence electrons with the outer hydrogens. So we have a total of four electrons, one from each atom in this arrangement and I'm going to put a little red kind of parentheses around this center with our two boons and our two central hydrogens to kind of focus on this unit here. So I've got four electrons in this area being shared and I've got these three atoms that each electron pair is being shared among. So imagine it's gonna draw two dots up on the top and two dots on the bottom. So these pair of electrons are being delocalized among the three atoms, the two boons and the hydrogen. So those types of bonds we'd call three center two electron bonds. Well, with this structure, we have enough electrons for the whole arrangement. 12 electrons, we use 2468 electrons in our normal usual two center, two electron bonds, we have four electrons left being shared in those three center two electron bonds. So that's why this is the normal structure of boron as a hydride. Since gallium is in the same group with three valence electrons, we can assume that it will form a similar bridge structure with these three center, two electron bonds only with gallium replacing one of the boons. So let's draw that we have our atom of gallium are atom of bo on and then they have two hydrogen atoms as a bridge between them. And then of course, the remaining four hydrogens as outer hydrogens and just as in our B two H six, our molecule would have these outer bonds being two center, two electron and the inner bonds being three center two electron bonds. So again, our plausible structure right here, four bgah six is this structure with two hydrogens bridging the gallium and the boron and then four outer hydrogens. See you in the next video.