BF3 Structure and Hybridization:
BF3 has a trigonal planar molecular geometry with the boron atom at the center and three fluorine atoms attached to it. The boron atom in BF3 undergoes sp2 hybridization, where one 2s and two 2p orbitals combine to form three sp2 hybrid orbitals. These hybrid orbitals form sigma bonds with the three fluorine atoms, resulting in a stable molecule.

BF4- Structure and Hybridization:
BF4- has a tetrahedral molecular geometry with the boron atom at the center and four fluorine atoms attached to it. The boron atom in BF4- undergoes sp3 hybridization, where one 2s and three 2p orbitals combine to form four sp3 hybrid orbitals. These hybrid orbitals form sigma bonds with the four fluorine atoms, resulting in a stable anion.

B-F Bond Length Comparison:
In BF3, the bond length between boron and fluorine is shorter compared to BF4-. This is because in BF3, the boron atom is in a trigonal planar geometry, leading to a stronger bond due to the closer arrangement of atoms. On the other hand, in BF4-, the boron atom is in a tetrahedral geometry, resulting in longer bond lengths due to the more spread out arrangement of atoms.
In conclusion, the hybridization of the central boron atom in BF3 is sp2, while in BF4- it is sp3. The B-F bond length in BF3 is shorter than in BF4- due to the different molecular geometries of the two compounds.