Boron Trifluoride Conversion Analysis

The question asks about the changes accompanying the conversion of boron trifluoride ($BF_3$) to tetrafluoroborate. This typically refers to the formation of the $[BF_4]^-$ ion from the $BF_3$ molecule, often via reaction with a fluoride source ($F^-$).

• Initial Molecule: Boron trifluoride ($BF_3$) has a trigonal planar geometry. The boron atom is $sp^2$ hybridized and has an incomplete octet. Its point group symmetry is $D_{3h}$.
• Final Ion: Tetrafluoroborate ($[BF_4]^-$) has a tetrahedral geometry. The boron atom is $sp^3$ hybridized and has a complete octet. Its point group symmetry is $T_d$.

Symmetry Changes: $BF_3$ vs $[BF_4]^-$

The conversion involves a change in geometry from trigonal planar ($D_{3h}$) to tetrahedral ($T_d$). Both geometries exhibit high symmetry. However, the formation of the stable, octet-complete tetrahedral ion ($[BF_4]^-$) from the electron-deficient trigonal planar molecule ($BF_3$) is associated with an increase in symmetry in the context of achieving a more stable and complete electronic configuration and geometric arrangement.

Bond Length Changes Observed

The hybridization change affects the B-F bond length:

• In $BF_3$, the $sp^2$ hybridized boron forms shorter B-F bonds (approximately 1.30 Å).
• In $[BF_4]^-$, the $sp^3$ hybridized boron forms longer B-F bonds (approximately 1.35 Å).

This indicates a bond elongation occurs during the conversion.

Conclusion on Conversion Effects

Combining the observations:

• Symmetry: Increases.
• B-F Bond: Elongates.

Therefore, the conversion of boron trifluoride to tetrafluoroborate is accompanied by an increase in symmetry and bond elongation.