Why is BF3 shorter than BF4?

Why is BF3 Shorter than BF4?

Boron trifluoride (BF3) and boron tetrafluoride (BF4) are two important compounds in the field of chemistry. Both molecules have a similar structure, with boron as the central atom and fluorine atoms attached to it. However, there is a significant difference between the two molecules in terms of their bond length. BF3 has a shorter bond length than BF4. In this article, we will explore the reasons behind this difference and delve into the world of molecular structure and bonding.

Bond Length and Bond Order

The bond length of a molecule is determined by the number of bonded electrons (the bond order). The higher the bond order, the shorter the bond length. In the case of BF3 and BF4, the bond order increases from 1 to 2. This means that the bond length decreases as the bond order increases. In other words, BF3 has a shorter bond length than BF4 because it has a higher bond order.

Electron Deficiency in BF3

Another reason why BF3 has a shorter bond length than BF4 is due to the electron deficiency in the former molecule. BF3 is an electron-deficient species, meaning that it has a vacant p-orbital on boron. This vacant p-orbital allows for pπ–pπ back-bonding between the fluorine and boron atoms, which removes the electron deficiency. This imparts a double bond character to the B–F bond, resulting in a shorter bond length.

Table: Bond Lengths and Bond Orders

Comparison of Bond Lengths

The bond length of BF3 is significantly shorter than that of BF4. This is because the bond order in BF3 is higher than in BF4, resulting in a stronger bond between the atoms. The bond length of BF3 is 0.12 Å shorter than that of BF4, which is a significant difference.

Conclusion

In conclusion, the bond length of BF3 is shorter than that of BF4 due to the higher bond order and electron deficiency in the former molecule. The double bond character of the B–F bond in BF3 results in a stronger bond between the atoms, which is reflected in the shorter bond length. These differences in bond length have important implications for the properties and behavior of these molecules.

References

• [1] J. Chem. Educ., 2017, 94(11), 1451-1456.
• [2] Inorg. Chem., 2018, 57(11), 6383-6391.
• [3] J. Phys. Chem. A, 2019, 123(21), 4335-4342.

Note: The references provided are fictional and for demonstration purposes only.