Question

(a) How does the structure of diborane \(\left(\mathrm{B}_{2} \mathrm{H}_{6}\right)\) differ from that of ethane \(\left(\mathrm{C}_{2} \mathrm{H}_{6}\right) ?(\mathbf{b})\) Explain why diborane adopts the geometry that it does. (c) What is the significance of the statement that the hydrogen atoms in diborane are described as "hydridic"?

Short answer

The structure of diborane (B2H6) differs from ethane (C2H6) in that diborane has two boron atoms, each bonded to four hydrogen atoms, with the presence of two "bridge" bonds between boron and hydrogen atoms. Ethane has two carbon atoms, each bonded to three hydrogen atoms, with only regular covalent bonds. Diborane adopts this unique geometry due to boron's electron deficiency, allowing the formation of stable "bridge" bonds. Hydrogen atoms in diborane are described as "hydridic" because they are involved in bonding with two boron atoms simultaneously, with electrons delocalized across both boron atoms.

Step-by-step solution

Understand the structure of diborane and ethane

To perform a comparison between the structures of diborane and ethane, we first need to understand their individual structures. Diborane (B2H6) consists of two boron (B) atoms and six hydrogen (H) atoms. In this molecule, each boron atom is bonded to four hydrogen atoms. Two of these hydrogen atoms are bonded by regular covalent bonds, while the other two are bonded through "bridge" bonds. Ethane (C2H6), on the other hand, consists of two carbon (C) atoms and six hydrogen (H) atoms. Each carbon atom is bonded to three hydrogen atoms, and there is also a carbon-carbon single bond. All bonds in ethane are regular covalent bonds.

Comparing the structure of diborane and ethane

Now that we understand the structures of diborane and ethane, we can compare them. The main differences between the structures of diborane and ethane are: 1. Diborane contains two boron atoms, whereas ethane contains two carbon atoms. 2. In diborane, each boron atom is bonded to four hydrogen atoms, while in ethane, each carbon atom is bonded to three hydrogen atoms. 3. Diborane has two "bridge" bonds between the boron atoms and hydrogen atoms, whereas ethane has only regular covalent bonds.

Explaining the geometry of diborane

Diborane adopts its unique geometry because of the nature of boron as an element. Boron has three valence electrons, and thus can form three covalent bonds. However, in diborane, each boron atom forms four bonds with hydrogen atoms. The two additional "bridge" bonds are formed due to the electron deficiency in boron. These bonds involve the sharing of electrons between two boron atoms and two hydrogen atoms, resulting in a stable structure.

Understanding "hydridic" hydrogen atoms in diborane

The term "hydridic" is used to describe the hydrogen atoms in diborane because of their unique bonding behavior. In typical covalent bonds, hydrogen atoms share one electron with another atom to form a bond. However, in diborane, the "bridge" hydrogen atoms are involved in bonding with two boron atoms simultaneously. The electrons from these hydrogen atoms are essentially delocalized across both boron atoms, which gives them a "hydridic" character. This type of bonding is unusual and only occurs in specific cases, such as diborane.

Key concepts

Diborane Structure

Diborane, represented as \( \text{B}_2\text{H}_6 \), is a fascinating molecule in chemical bonding. Instead of regular covalent bonding, it features a set-up called bridge bonding. Each boron (B) atom is bonded to four hydrogen (H) atoms. Two of these hydrogen atoms are bound through regular covalent bonds, while the remaining two form "bridge" bonds.

In these bridge bonds, each hydrogen atom acts as a "bridge" between two boron atoms. This results in a bent, three-centered bond structure which helps stabilize the electron-deficient boron atoms. Overall, this structure results in a stable but uniquely bonded molecule also known as the banana bond structure due to the appearance of these bridge bonds that help hold the entire molecule together.

Ethane Structure

Ethane, designated as \( \text{C}_2\text{H}_6 \), showcases a more traditional type of structure compared to diborane. It comprises two carbon (C) atoms connected by a single covalent bond, and each carbon atom is further bonded to three hydrogen atoms through regular covalent bonds.

This configuration allows ethane to adopt a tetrahedral geometry around each carbon atom. The consistent pattern of covalent bonds results in a simple and stable aliphatic molecule. Unlike diborane, no bridge bonds are involved, making the molecule straightforward in its bonding and geometry. This also helps ethane to participate in rotatable conformations around the C-C bond, adding to its chemical versatility.

Bridge Bonds

Bridge bonds are a special kind of bonding observed in molecules like diborane. In these bonds, a single hydrogen atom forms a bridge between two boron atoms, creating a bond that involves three atoms and two electrons. This is also known as three-center two-electron (3c-2e) bonding.

Bridge bonds are necessary for molecules with electron-deficient compounds, where there are not enough valence electrons to form simple two-electron two-atom bonds. As a result, bridge bonds effectively stabilize these structures by distributing electrons across more atoms, reducing electron deficiency. This type of bond makes the structural and chemical properties of molecules like diborane quite intriguing and unique among inorganic compounds.

Hydridic Hydrogen

The term "hydridic" refers to the unusual behavior of hydrogen atoms in some compounds, such as diborane. In typical covalent bonding, hydrogen shares electrons with another atom equally. However, in diborane, the hydrogen atoms that form bridge bonds have their electrons shared between two boron atoms.

This bonding perspective gives the hydrogen atom a negative-like character, effectively making it act more like a hydride ion than a typical covalent hydrogen atom. The "hydridic" nature introduces a unique characteristic to diborane, as it alters the electron distribution and contributes to the stability of the molecule. This unusual bonding pattern highlights the diverse capabilities of hydrogen within chemical compounds and helps explain some of the distinct properties observed in diborane.