Lewis acidity in the boron halides

The boron halides have the expected formula, BX3. Boron trifluoride can be made by heating boric oxide, calcium fluoride and concentrated sulfuric acid  

Now both experimental measurements and VSEPR theory agree that the BX3 molecules in these substances are trigonal planar (Structure 9.2). As noted in Section 9.1.1, boron then has six outer electrons. 

The trihalide can act as a Lewis acid, acquiring a share in the non-bonded pair of a Lewis base, such as ammonia or trimethylamine for example  

In the white solid product, the geometry around both the donor nitrogen atom and the acceptor boron atom is tetrahedral (Structure 9.3) the structure of the complex is analogous to that of a substituted ethane, C2H6. It is isoelectronic with (CH3)3C—CX3. The Lewis base may also be a halide ion thus when BF3 is heated with solid KF, the tetrahedral tetrafluoroborate ion (Structure 9.4) is formed  

This Section has shown you two ways in which a trivalent boron atom can increase the number of electrons that it shares with other atoms boron compounds can act as Lewis acids, or, if a boron atom is bound to halogen or oxygen atoms with nonbonding electrons, it can participate in 7t bonding. 

---