Why is BF3 more stable than BH3?

Answered by Randy McIntyre

BF3 is more stable than BH3 due to the difference in electron density and the resulting electronic structure of the two molecules. The stability of a molecule is determined by its ability to maintain its structure and resist undergoing chemical reactions. In the case of BF3 and BH3, the stability is influenced by the electronegativity difference between boron (B) and fluorine (F) or hydrogen (H), as well as the molecular geometry.

One of the key factors contributing to the stability of BF3 is the electron-withdrawing nature of fluorine. Fluorine is highly electronegative, meaning it has a strong tendency to attract electrons towards itself. In BF3, the fluorine atoms pull the shared electron pairs towards themselves, creating a partial positive charge on the boron atom. This electron-withdrawing effect makes the boron atom in BF3 more electron deficient or electron poor.

On the other hand, BH3 does not have as strong of an electron-withdrawing effect because hydrogen is less electronegative than fluorine. This results in a less electron-poor boron atom in BH3 compared to BF3. The lower electron density around the boron atom in BH3 makes it less stable.

The higher stability of BF3 can also be attributed to its molecular geometry. BF3 adopts a trigonal planar geometry, with the boron atom at the center and the three fluorine atoms surrounding it in a flat plane. This arrangement allows for maximum separation between the electron pairs and minimizes repulsion between them, resulting in a more stable structure.

In contrast, BH3 has a pyramidal geometry, with the boron atom at the apex of a pyramid and the three hydrogen atoms at the base. This geometry leads to greater electron pair repulsion and less stability compared to the planar geometry of BF3.

To summarize, BF3 is more stable than BH3 due to the stronger electron-withdrawing effect of fluorine compared to hydrogen, resulting in a more electron-poor boron atom. Additionally, the trigonal planar geometry of BF3 minimizes electron pair repulsion and enhances stability.