Boron (B) belongs to the second period of the periodic table. Its valence shell contains only four orbitals (one 2s and three 2p orbitals). Due to the absence of vacant d-orbitals in its valence shell, boron cannot expand its octet or coordination number beyond 4. Therefore, it forms [BF4]- but cannot form the [MF6]3- (e.g., [BF6]3-) ion. The other elements (Al, Ga, In) belong to higher periods and have vacant d-orbitals, allowing them to expand their coordination number to 6 and form [MF6]3- ions.