Why `B-X` bond distance in `BX_(3)` is shorter than theoretically expected value ?

Explanation

`B-X` bond distance in `BX_(3)` is shorter than theoretically expected due to `p pi-p pi` back bonding form the filled `np` orbital of `X` on the vacant `2p` orbital of the central `B` atom.

Hence, `B-X` bond in `BX_(3)` acquires some double bond character and consequently, `B-X` bond is shorter than expected.