Question

Explain structures of diborane and boric acid.

Short answer

Diborane has a bridging structure with 3c-2e bonds, while boric acid has a planar triangular structure with three OH groups.

Step-by-step solution

Understanding Diborane's Molecular Formula

Diborane is a chemical compound with the molecular formula \( B_2H_6 \). This formula suggests it has two boron (B) atoms and six hydrogen (H) atoms.

Exploring Diborane's Structure

In diborane, the two boron atoms form a bridge using two hydrogen atoms, creating a unique bonding situation. The structure is often described as having two three-center two-electron (3c-2e) bonds. This is unusual because it involves three atoms sharing two electrons. The remaining four hydrogen atoms are terminal, each bonded to a single boron atom.

Visualizing Boric Acid's Chemical Structure

Boric acid has the chemical formula \( B(OH)_3 \). It consists of one boron atom bonded to three hydroxyl groups (OH). The structure forms a planar triangular shape around the boron atom because it uses sp2 hybridization. Every B-O bond is covalent, and boric acid often forms hydrogen bonds with nearby molecules.

Key concepts

Diborane

To understand diborane, let's start with its molecular formula, which is \( B_2H_6 \). This indicates that diborane consists of two boron atoms and six hydrogen atoms. The bonding in diborane is quite adventurous. Instead of forming conventional 2-centre 2-electron bonds, diborane is characterized by two unique 3-centre 2-electron (3c-2e) bonds. This means that two boron atoms and two of the hydrogen atoms come together in a way that allows each hydrogen to link both borons.
The two hydrogen atoms in the center form a bridge between the boron atoms, creating these unusual bonds. Hence, diborane has what are known as 'banana bonds' or 'hydrogen bridges.' Meanwhile, the remaining four hydrogen atoms are located at the ends and are each singly bonded to the boron atoms in a more typical fashion. This arrangement gives diborane its distinct and interesting molecular structure.

Boric Acid

Boric acid is a well-known compound with the formula \( B(OH)_3 \). In this compound, boron is centrally bonded to three hydroxyl groups (OH). The arrangement of these groups forms a planar triangular shape. This structure is due to the sp2 hybridization of the boron atom, which creates a flat molecule.
The bonds between boron and oxygen in boric acid are covalent. One of the key features of boric acid is its ability to form hydrogen bonds with adjacent molecules. These hydrogen bonds occur between the hydroxyl groups of the boric acid and surrounding molecules. This property is vital in explaining boric acid's solubility behavior and its tendency to crystallize in thin sheets.

• Planar shape due to sp2 hybridization
• Strong hydrogen bonding ability
• Covalent B-O bonds

Chemical Bonding

Chemical bonding in molecules refers to how atoms connect and interact within a compound. There are several types of chemical bonds, but the bonds in diborane and boric acid show unique characteristics.
In diborane, the distinctive feature is the 3-centre 2-electron (3c-2e) bonds, where three atoms share two electrons. This is relatively rare and provides an insight into the flexibility of chemical bonding, showing how unconventional connections delve into multi-center scenarios.
In contrast, the bonding in boric acid is dominated by covalent bonds. Covalent bonds involve the sharing of electron pairs between atoms. These bonds are crucial for the stability and formation of molecular structures in many compounds including boric acid.

Hybridization

Hybridization is a concept that explains the arrangement of atoms in molecules by the mixing of atomic orbitals to form new hybrid orbitals. This process is responsible for the shapes and geometry of molecules.
In diborane, hybridization helps form its complex bond arrangement. The boron atoms exhibit sp3 hybridization, providing the necessary geometry for the formation of both terminal and bridge bonds.
In boric acid, boron atoms undergo sp2 hybridization, which leads to a planar triangle layout where each boron atom forms three sigma bonds with hydroxyl groups.

• sp3 hybridization in diborane - supports bridging hydrogen bonds
• sp2 hybridization in boric acid - leads to planar structures

Hybridization is a powerful tool to predict molecular shape and bonding angles, making it essential for understanding molecules like diborane and boric acid.