Electron defficient molecules Electron defficiency is a common feature in some class of compounds of Group 13 elements (B,Al,Ga). In some covalent compounds of Be too the same nature prevails. To compansate this electron defficiency they undergo dimerization forming bridges('3c 2e' or coordination) or in some cases they get stability through backbonding (halides of group 13 elements). Hydrides of them are known to stay in dimeric form through bridging.
Halides of B are found to remain in monomeric form due to backbonding where as 'Be, Al'-halides highly favour bridging structures through coordination of halide. Why is such difference? In Be,Al, vacant p orbital is present too to favour backbonding.
Then why Be,Al favour dimerization (or polymerization) over backbonding in their halides?
I believe this may be a duplicate of this question:
Why is BCl3 a monomer whereas AlCl3 exists as a dimer?
From what I remember, the stronger the backbond, the smaller the tendency to undergo dimerisation, however, way back in the dark ages when I was a student and asked my second-year chem prof, his answer was that the simple reason why none of the tri-halides of boron undergo dimerisation is steric hinderance - i.e. the boron atom is just too small to accommodate the fourth larger Cl/Br/I atom. He then pointed me to several sets of further reading.
However, you will probably find a better answer at one of the following links:

Mulliken J. Chem. Phys. 3, 635 (1935); https://doi.org/10.1063/1.1749567
Lappert, et.al. Chem. Soc., Faraday Trans. 2, 1976,72, 539-55 https://doi.org/10.1039/F29767200539