Answer:
This is due to \[p\pi -p\pi \] back
bonding of the fully filled p-orbital of halogen (X) into the empty p-orbital
of boron (Fig. 11.5, page 11/13). As a result, B-X bond has some double bond
character (Fig. 11.6, page 11/13)and hence B-X bond is shorter than expected.
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