Question

In diborane, (a) four bridged hydrogen atoms and two terminal hydrogen atoms are present (b) two bridged hydrogen atoms and four terminal hydrogen atoms are present (c) three bridged hydrogen atoms and three terminal hydrogen atoms are present (d) there are no bridged hydrogen atoms in diborane, only hydrogen bonds are present.

Short answer

(a) four bridged hydrogen atoms and two terminal hydrogen atoms are present in diborane.

Step-by-step solution

Understanding the Structure of Diborane

Diborane, which has a molecular formula of B2H6, consists of two boron (B) atoms and six hydrogen (H) atoms. The structure features both bridged and terminal hydrogen atoms. Bridged hydrogens are those that are bonded to both boron atoms simultaneously, whereas terminal hydrogens are bonded to only one boron atom.

Identifying Bridged and Terminal Hydrogens

In the case of diborane, there are four hydrogen atoms that serve as bridges between the two boron atoms. These are known as 'banana bonds' due to their curved shape. Additionally, each boron atom is bonded to one hydrogen atom that is not involved in bridging, making a total of two terminal hydrogen atoms.

Determining the Correct Statement

By analyzing the structure of diborane, we can see that there are four bridged hydrogen atoms and two terminal hydrogen atoms as indicated by the molecular structure and bonding patterns.

Key concepts

Bridged Hydrogen Atoms in Diborane

Understanding the unique nature of bridged hydrogen atoms is crucial when analyzing diborane's structure. In this unusual type of bonding, a hydrogen atom forms a link between two boron atoms, effectively being bonded to both simultaneously. This forms a bridge-like configuration that is central to the structure of diborane. Remarkably, these hydrogen atoms participate in a type of bonding known as three-center two-electron bonds. The intriguing aspect is that two boron atoms share a pair of electrons with one hydrogen atom, a divergence from the typical one electron from each atom seen in conventional covalent bonds.

These bridged hydrogens are crucial for the stability of the diborane molecule. Due to their importance in the molecular geometry, they contribute significantly to the electron-deficiency of the boron atoms they connect. This arrangement is not randomly formed, but rather, is a clever way in which nature overcomes the electron deficiency of boron, an element notorious for its incomplete octet.

Terminal Hydrogen Atoms in Diborane

In addition to the bridged hydrogen atoms, diborane also features terminal hydrogen atoms. These atoms are only bonded to a single boron atom, contrasting the multi-boron connectivity of their bridged counterparts. Each of the two boron atoms in the molecule has one such terminal hydrogen atom, completing their coordination sphere. The bond these terminal hydrogens form with the boron is a regular two-center two-electron bond, much like the bonds found in simpler molecules like methane (CH₄).

These terminal hydrogen atoms lend diborane part of its chemical reactivity. Unlike the bridged hydrogens that are tucked within the molecule's structure, these are relatively more exposed and therefore, more chemically active. This is why termination reactions typically happen at these sites, showcasing the distinctive roles that different types of hydrogens play within diborane's architecture.

Molecular Formula of Diborane

Exploring the molecular formula of B₂H₆ reveals the underlying simplicity behind the seemingly complex structure of diborane. The formula indicates the presence of two boron atoms and six hydrogen atoms within the molecule. However, the mere enumeration of atoms does not do justice in conveying the intricate bonding found in diborane. The four hydrogen atoms that connect as bridges and the two that exist as terminal atoms construct the full image of the molecule's actual geometry.

The molecular formula serves as the fundamental identifier for the compound, often used as a starting point for deeper chemical analysis. It is the snapshots taken from structural studies, like X-ray crystallography or electron diffraction, combined with theoretical calculations, which reveal how these atoms are spatially arranged and connected. This portrays a more complete picture, bridging the gap between the simple molecular formula and the complex structural arrangement that is characteristic of diborane.