Problem 16 - Chemistry in 30 days

Question

Which of the following molecules forms 2 -electron 3 -centred bonds? (a) \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\) (b) \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{B}_{2} \mathrm{H}_{6}\) (d) \(\mathrm{BH}_{3}\)

Step-by-Step Solution

Answer

The molecule \\( ext{B}_2 ext{H}_6\\) forms 2-electron, 3-centered bonds.

Step 1: Identify 2-Electron 3-Centered Bonds

2-electron, 3-centered bonds are a type of electron-deficient bonding, often found in molecules with boron or aluminum when they form 'bridged' structures. This type of bond is prevalent in boranes like \( ext{B}_2 ext{H}_6\), where hydrogen atoms bridge between two boron atoms.

Step 2: Assess Each Molecule

Examine each molecule: (a) \( ext{Al}_2 ext{Cl}_6\) forms bridged structures but with different bonding.(b) \( ext{Hg}_2 ext{Cl}_2\) does not typically exhibit 2-electron, 3-centered bonding.(c) \( ext{B}_2 ext{H}_6\) is known for having 2-electron, 3-centered bonds with hydrogen atoms bridging boron atoms.(d) \( ext{BH}_3\) typically exists as a monomer but is highly reactive due to an incomplete octet, often forming \( ext{B}_2 ext{H}_6\).

Step 3: Conclusion

The molecule \( ext{B}_2 ext{H}_6\) is the only one from the given options that forms 2-electron 3-centered bonds between the hydrogen and boron atoms in its dimeric form.

Key Concepts

2-electron 3-centered bonds

2-electron 3-centered bonds are an intriguing feature of electron-deficient compounds. These bonds appear in molecules where two electrons are shared among three atoms. This type of bonding is atypical, since we often expect bonds to involve two atoms sharing electron pairs. However, certain elements like boron, which naturally have fewer valence electrons, must resort to unconventional bonding to satisfy their structural needs.
In the 2-electron 3-centered bond scenario, the bridge atom (often hydrogen) shares its electrons with two other atoms, like boron, effectively creating a bridge between them. This type of bonding strategy is common in boranes, such as diborane ( ext{B}_2 ext{H}_6"). Here, the hydrogen bridge facilitates a stable structure, despite the lack of a full complement of shared electron pairs per bond.
Understanding this kind of bonding helps in grasping the unique structural aspects of certain chemicals, where typical electron-sharing rules do not apply. It’s an excellent example of how nature finds resourceful ways to maintain chemical stability.

boranes

Boranes are fascinating compounds primarily composed of boron and hydrogen. They are renowned for their unique bonding arrangements due to boron's inability to complete its octet in a conventional manner. One of the most remarkable features of boranes is their usage of 2-electron 3-centered bonds.
In boranes, like diborane ( ext{B}_2 ext{H}_6"):

• Hydrogen atoms serve as bridges between boron atoms.
• These structures display mutual electron sharing across three centers, thanks to the bridging hydrogen atoms.
• This allows boranes to achieve stability despite low electron counts.

Boranes are not only of academic interest; they have practical applications too. Understanding these allows chemists to synthesize novel materials and catalyze various reactions. The study of boranes can illuminate broader principles of chemistry, particularly for those exploring unconventional forms of electron bonding.

molecular structure

The molecular structure of compounds with electron-deficient bonding can be quite unusual and is essential for understanding their chemical behavior. In electron-deficient compounds like ext{B}_2 ext{H}_6", the structural design allows the molecule to achieve stability beyond typical bonding rules.
The insight into the molecular structure of boranes reveals:

• The importance of space and geometry in stabilizing the entire molecule.
• Instead of every atom having a complete octet, strategic electron sharing maximizes stability.
• The role of bridging atoms, like hydrogen, in connecting other atoms which are electron-deficient.

By studying such molecular structures, we not only appreciate the cleverness of chemical designs but also gain profound insights into how nature can optimize limited resources. This knowledge is invaluable for students and professionals as it enhances the comprehension of more complex systems and aids in the development of innovative materials.