Question

Suggest the reason why the $\text{B - F}$ bond length in $\text{B}{{\text{F}}_{\text{3}}}$ is $\left( \text{130 pm} \right)$ while that in $\text{B}{{\text{F}}^{\text{-}}}_{4}$ $\left( \text{143 pm} \right)$ differ?

Answer 1

When two or more than two orbitals hybridize with each other to form hybrid orbitals then the bond length and the bond strength depends on the nature of the hybrid orbitals and the percentage of the “s-character” and the “p-character” of the orbitals.

Complete stepwise Solution
The s and the p orbitals can hybridize with each other to form three types of hybrid orbitals: $\text{sp}$ hybridization, $\text{s}{{\text{p}}^{\text{2}}}$ hybridization, and $\text{s}{{\text{p}}^{3}}$ hybridization.
In the $\text{sp}$ hybrid orbitals, the percentage of the s-character is $50$ 50%]
In the $\text{s}{{\text{p}}^{\text{2}}}$ hybrid orbitals, the percentage of the s-character is $33.33$ 66.67\text{ }%]
In the $\text{s}{{\text{p}}^{3}}$ hybrid orbitals, the percentage of the s-character is $25$ 75%].
Now, the s-orbitals are spherical in shape while the p-orbitals are dumbbell shaped. Hence the s-orbitals, having equal distribution of electrons on all sides, form better covalent bonds that the p-orbitals which have the electron density located only on the top and the bottom of the molecular plane. Therefore, the $\text{sp}$ hybrid orbitals make the stronger bonds due to higher s-character than the $\text{s}{{\text{p}}^{\text{2}}}$ hybrid orbitals which make stronger bonds than $\text{s}{{\text{p}}^{3}}$ hybrid orbitals.
In the $\text{B}{{\text{F}}_{\text{3}}}$ molecules, the hybridization of the boron atom is $\text{s}{{\text{p}}^{\text{2}}}$ while that in the $\text{B}{{\text{F}}^{\text{-}}}_{4}$ ions is $\text{s}{{\text{p}}^{3}}$ .
Hence, the $\text{B}{{\text{F}}_{\text{3}}}$ molecules have stronger $\text{B - F}$ bonds and shorter bond lengths as compared to the $\text{B - F}$ bonds in $\text{B}{{\text{F}}^{\text{-}}}_{4}$ molecules.

Note
The hybridization is the phenomenon that involves intermixing of the atomic orbitals to have mixed orbitals with the properties of the orbitals involved in the bond formation. The number of the hybrid orbitals formed is the same as the number of the orbitals involved in bonding.