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?