Boron-Boron bonds

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



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


Anyone can explained to me why is $\ce{B-B}$ bonds can be formed in the first place?




As mentioned in the comments, *why not*? A boron-boron bond will form as long as there is an increase in stability of the system. Forming a bond requires energy, but there is a lot of energy released once it gets formed. The feasibility of a bond depends which energy is greater.


One example of where a boron-boron bond occurs is in [Aluminium diboride](http://en.wikipedia.org/wiki/Aluminium_diboride) / [Magnesium diboride](http://en.wikipedia.org/wiki/Magnesium_diboride)


![enter image description here](https://i.stack.imgur.com/Hw0Ja.png)


Here, you have graphite-like "sheets" of boron (pink), with aluminium layers between them.


This will probably be quite stable, due to [delocalization](http://en.wikipedia.org/wiki/Delocalized_electron). 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](http://en.wikipedia.org/wiki/Lanthanum_hexaboride) and various [Yttrium borides](http://en.wikipedia.org/wiki/Yttrium_borides). These are probably very stable due to the crystal structure (though delocalization may be a factor as well)


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