Diborane is supposed to be a dimer of BH<sub>3</sub>, which is an electron-deficient species so it needs to look for a way to gain some respite from the drought of electron density. So it adjusts for this 3c-2e bonded system where it shares the B-H σ-bond electron density and shares it to the Boron atom of another BH<sub>3</sub> unit, forming a dimer.

[![enter image description here][1]][1]

The 'bridging' bond has a bond order of 0.5 as @MollyCooL stated, which means it is certainly a weaker bond than the terminal ones, which have 'unity' bond order. However, in the case of alkylation reaction, the reacting unit being the actual species BH<sub>3</sub>, the dimer has to break its bridges, let BH<sub>3</sub> react, and then when the product is formed, look for a way again to stabilize itself by forming that bridge bond again.

When we look at the final product, it seems like the bridge hasn't broken at all. But it has only reformed. 

Here, the bond length does run in an inverse relation with the bond strength as per the image data (Wikipedia). The bridge is indeed weaker than the terminal bond.
However it may not be true in all cases. (Thanks Martin for correcting)


P.S. I am a new contributor and not very experienced with the subject so please correct me if I am wrong.


  [1]: https://i.sstatic.net/gJyvQ.png