Question

The structure of diborane \(\left(\mathrm{B}_{2} \mathrm{H}_{6}\right)\) contains (a) Four 2c-2e bonds and two \(3 \mathrm{c}-2 \mathrm{e}\) bonds (b) Two \(2 \mathrm{c}-2 \mathrm{e}\) bonds and four \(3 \mathrm{c}-2 \mathrm{e}\) bonds (c) Two \(2 \mathrm{c}-2 \mathrm{e}\) bonds and two \(3 \mathrm{c}-3 \mathrm{e}\) bonds (d) Four \(2 \mathrm{c}-2 \mathrm{e}\) bonds and four \(3 \mathrm{c}-2 \mathrm{e}\) bonds

Short answer

(a) Four 2c-2e bonds and two 3c-2e bonds.

Step-by-step solution

Understand the Bond Terms

In chemistry, a 2c-2e bond refers to a 'two-center, two-electron' bond, which is a standard covalent bond. A 3c-2e bond, or 'three-center, two-electron' bond, is a type of bond where three atoms share two electrons. This is also known as a 'banana bond'.

Mcaborane bonds analysis

Diborane (B_2H_6) consists of four terminal hydrogen atoms each forming standard covalent (2c-2e) bonds with the boron atoms. This accounts for the four 2c-2e bonds.

Identify bridge bonds

Diborane contains two bridging hydrogen atoms that form 3c-2e bonds, each connecting two boron atoms and one bridging hydrogen atom. These are the two banana bonds in the molecule.

Choose the Correct Answer

We have identified four 2c-2e bonds from the terminal B-H bonds and two 3c-2e bonds from the bridging hydrogens. Thus, the correct answer is (a) Four 2c-2e bonds and two 3c-2e bonds.

Key concepts

2c-2e bond

The term "2c-2e bond" stands for a "two-center, two-electron" bond. This is the most common type of covalent bond we encounter in chemistry. In a 2c-2e bond, two atoms share a pair of electrons. This sharing allows each atom to achieve a stable electronic configuration. Imagine it as a handshake where each atom brings one electron to the table, making both of them happy and stable.
In the context of diborane \((\mathrm{B}_{2}\mathrm{H}_{6})\), the four terminal hydrogen atoms form 2c-2e bonds with the two boron atoms. These bonds are straightforward, similar to what you find in simple molecules like hydrogen gas \((\mathrm{H}_{2})\).
This stable bond formation ensures that both hydrogen and boron achieve a level of electron stability.

• 2c-2e bonds are typical covalent bonds.
• Shared pair of electrons leads to bond stability.
• In diborane, 2c-2e bonds occur between boron and terminal hydrogens.

3c-2e bond

A "3c-2e bond" or "three-center, two-electron" bond is more complex than the usual covalent bond. Here, three atoms share just two electrons, a unique feature in some molecules including diborane.
These bonds are also called "banana bonds" because they form a banana-like shape between the atoms involved.
In diborane, the 3c-2e bonds involve two boron atoms and one bridging hydrogen atom. This sharing of electrons over three centers allows the molecule to maintain stability despite having fewer electrons than typical shared-electron bonds.
These bonds are less common but essential for maintaining the molecular structure when traditional 2c-2e bridging isn't possible.

• Involves three atoms sharing two electrons.
• Creates specialized banana-shaped bonds.
• Found in complex molecules like diborane.
• Maintains molecule stability with fewer electrons.

Structure of diborane

Diborane \((\mathrm{B}_{2}\mathrm{H}_{6})\) is a fascinating compound due to its unique structure. It is essentially made up of eight atoms: two boron atoms and six hydrogen atoms.
The structure can be visualized as two \(\mathrm{BH}_{3}\) units. These units are bridged by two hydrogen atoms, which form the notable 3c-2e bonds.
This structure is necessary due to the deficiency of electrons to form normal covalent bonds between all atoms involved. The four terminal hydrogens are connected by the conventional 2c-2e bonds to the boron atoms, while the two bridging hydrogens connect to both boron atoms and form the less typical 3c-2e bonds. This particular arrangement allows diborane to exist as a stable molecule despite electron deficiency.

• Diborane consists of 2 boron and 6 hydrogen atoms.
• Structure includes two \(\mathrm{BH}_{3}\) units linked by bridging hydrogens.
• Electron-deficient but stable due to its bonding.

Covalent bond

A covalent bond is a powerful component of chemistry, essential for forming molecules. It occurs when two atoms share a pair of electrons to achieve stable electron configurations. This sharing results in the attraction that keeps the atoms bonded together.
Covalent bonding can happen in several forms, such as single, double, or triple bonds, based on the number of electron pairs shared. There are variations like polar and non-polar covalent bonds.
In non-polar covalent bonds, electron sharing is equal, while in polar bonds, electrons are shared unequally, affecting molecule properties.
In diborane, covalent bonding in the form of 2c-2e and 3c-2e bonds is critical. While the 2c-2e bonds are typical covalent interactions, the 3c-2e bonds show the versatility of covalent bonding in accommodating special structural needs like in electron-deficient molecules.

• Covalent bonds involve electron pair sharing.
• Can be single, double, or triple based on shared pairs.
• Polar bonds involve unequal electron sharing.
• Essential for both simple and complex molecules.