Electron defficient molecules
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