Boranes: Structure, Bonding, and Thermochemistry

Boron Family: Boranes

Introduction to Boranes

Boranes are a class of compounds consisting of boron and hydrogen atoms. They are notable for their unique bonding and structural features, which are important in both inorganic and general chemistry.

• Boranes are highly reactive covalent hydrides composed only of boron and hydrogen atoms.

• The simplest borane is diborane (B2H6), which consists of two boron atoms connected by bridged hydrogens.

• Bridging Hydrogens: In diborane, hydrogen atoms can bridge two boron atoms via so-called 3-center, 2-electron bonds, which are a hallmark of electron-deficient compounds.

Note: The detailed bonding in boranes is typically covered in more advanced chemistry courses, but the basic structure and bonding concepts are essential for understanding their properties.

Structure of Diborane (B2H6)

• Diborane contains four terminal hydrogen atoms bonded directly to boron and two bridging hydrogen atoms that connect both boron atoms.

• The bonding in diborane involves both conventional 2-center, 2-electron bonds (B-H) and 3-center, 2-electron bonds (B-H-B bridges).

Diagram: (A structural diagram of diborane is referenced in the original material.)

Hybridization in Diborane

To determine the hybridization of boron atoms in diborane, consider the number of regions of electron density around each boron atom.

• Each boron atom in diborane forms four bonds: two terminal B-H bonds and two bridging B-H bonds.

• This arrangement suggests sp3 hybridization for each boron atom.

Example: Based on the structure of the diborane molecule, the hybridization of the boron atoms is sp3.

Thermochemistry of Diborane Formation

The formation of diborane from its elements involves several steps, and the enthalpy change can be calculated using Hess's Law and standard enthalpy values for related reactions.

• The overall reaction for the formation of diborane from its elements is:

$2B(s) + 3H_2(g) \rightarrow B_2H_6(g)$

• To determine the enthalpy change (ΔH) for this reaction, use the enthalpy values of related reactions and apply Hess's Law.

• Relevant reactions and their enthalpy changes are provided (see original material for specific values).

Key Formula:

$\Delta H_{\text{rxn}} = \sum \Delta H_{\text{f,products}} - \sum \Delta H_{\text{f,reactants}}$

Example: Given a set of reactions and their enthalpy changes, you can algebraically combine them to find the enthalpy of formation for diborane.

Summary Table: Key Features of Diborane

Feature	Description
Empirical Formula	B2H6
Bonding	4 terminal B-H bonds (2-center, 2-electron); 2 bridging B-H-B bonds (3-center, 2-electron)
Boron Hybridization	sp3
Reactivity	Highly reactive, especially with oxygen and water

Additional info: The concept of 3-center, 2-electron bonds is a key example of electron-deficient bonding, which is important in the chemistry of boron and other main group elements.