system
Given the following post, respond as if you're commenting on Reddit:
human
How does diborane (B2H6) exist?

The hydrogens having two bonds is really perplexing to me and it kinda breaks what we are taught at school about bonding and orbitals.
gpt
OK so what you were taught wasn't wrong, any more than say teaching someone stuff in mechanics and talking about perfect spheres etc is "wrong". It's just that you got a simplified version.

So firstly let's consider methane, CH4 - the Carbon 2s and 2p orbitals hybridise to form sp3 orbitals. This means the carbon can now have 4 equivalent bonds arranged in a tetrahedral structure, and together with the 4 H atoms, it now means that there are 8 electrons in the outer shell of the Carbon which makes it "happy", or technically it is now in a low energy state which makes it relatively stable because the 4 available orbitals are all full.

Hopefully you're with me so far.

Now Boron has 1 electron less than Carbon, so if it did the same thing: sp3 hybrid orbitals, 4 bonds to H atoms, it would have just 7 electrons, 3 from the Boron and 4 from the Hydrogens. That's not enough. But it gets around this with the 3 centre, 2 electron bonds, in which it effectively shares 2 electrons with another Boron atom via these "banana bonds".  Each Boron now effectively has 8 electrons in the outer shell, so just like methane, the structure is now stable.