The formation of stronger backbond ( $3\mathrm{p}\to 2\mathrm{p}$ overlap in $\mathrm{B}\mathrm{C}{\mathrm{l}}_3$ and weaker $3\mathrm{p}\to 3\mathrm{p}$ overlap in $\mathrm{A}\mathrm{l}\mathrm{C}{\mathrm{l}}_3$ ) reduces the electron deficiency of the central atom and thus dimerisation tendency is reduced. So $\mathrm{A}\mathrm{l}\mathrm{C}{\mathrm{l}}_3$ can form a dimer but $\mathrm{B}{\mathrm{F}}_3$ and $\mathrm{B}\mathrm{C}{\mathrm{l}}_3$ cannot.