# Boron (II) halides

I am reading up on boron in my notes and there is a certain paragraph that I do not understand

The tendency of B to form B-B bonds is less pronounced than that of carbon, its periodic table neighbor on the right, to form C-C bond, but it is much greater than that of the nearest element in the group, Al, to form Al-Al bonds

Anyone can explained to me why is B-B bonds can be formed in the first place?

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I agree with @F'x. A boron-boron bond will form because it is lower in energy than the two atoms separately (definition of a bond in all bonding theories). A better question is why the B-B bond is less common than the C-C bond, but more common than the Al-Al bond. –  Ben Norris Nov 9 '12 at 12:14

This will probably be quite stable, due to delocalization. The singly-filled aluminium $p$-orbitals will overlap with the empty $p$ orbitals of Boron, and will delocalize across the graphite-like sheet (I cannot confirm this, though). This will lead to a large increase in stability.
A few more compounds with $\ce{B-B}$ bonds are Lanthanum hexaboride and various Yttrium borides. These are probably very stable due to the crystal structure (though delocalization may be a factor as well)