Boron Chemistry Overview

Boron - Nonmetal - Electron Deficient

• Boron exists in several allotropes.
• Solid phases' crystal structures utilize the icosahedral B₁₂ unit (20-faced) as a recurring motif in boron chemistry.
• This unit has 12 vertices, 20 triangular faces, and 30 edges.
• Amorphous boron is a brown powder, while crystalline boron forms shiny black crystals.

Boron Bonding

• Boron is unique among elements due to having fewer electrons (1s², 2s² 2p¹) than atomic orbitals available for bonding.
• It forms covalent bonds (no metallic bonds) due to its small size and high ionization energy.
• This structural complexity leads to allotropic modifications and several allotropes.

Boron Hydrides - Diborane

• Diborane, B₂H₆ (diborane(6)), is a boron hydride.
• B atoms are typically attached to more than 3 atoms, and bridging hydrogen atoms are frequently present.
• Bonding in boron hydrides is not easily described using VB theory.
• The structure of B₂H₆ (D₂h symmetry) is shown in Figure 5.31.
• Key features: bridging H atoms are attached to two B atoms, each B atom is bonded to four H atoms, and BH bond lengths vary, suggesting different bonding types.
• B₂H₆ is described as electron-deficient. It's a dimer of BH₃ and possesses 12 valence electrons.

Structure of Diborane

• The formation of B-H-B bridges is depicted as in diagram 5.10.
• Terminal BH interactions are considered 2c-2e bonds.
• Bridging units are treated as 3c-2e bonding interactions.
• The 3c-2e interaction in half is expected to be weaker than a terminal 2c-2e bond (consistent with observed bond lengths)

Molecular Orbital (MO) of Diborane

• MO treatment provides insight into electron density distribution in diborane (B₂H₆).
• Using ligand group orbital approach, interactions between bridging H atoms and the B₂H₄ fragment can be considered, as seen in Figures below.
• The structure of B₂H₆ is broken down into H₂B-BH₂ and H-H fragments.
• The ligand group orbitals (LGOs) for the H-H fragment are considered.
• Six lowest-energy LGOs for the B₂H₄ unit, with nodal planes in the b₂u orbitals are examined.
• An important MO model conclusion is that boron-hydrogen bridge character is delocalized across all four bridging atoms within B₂H₆.
• Two of the MOs involve four electrons each. This is consistent with the 3c-2e B-H-B model.
• Terminal B-H bonds and bridging B-H-B bonds each contain two electrons.

Xenon Fluorides

• Xenon fluorides are synthesized by reacting xenon elements directly, usually within a nickel reaction vessel.
• The vessel is passivated with F₂ forming a NiF₂ coating to remove surface oxides, which react with xenon fluorides.
• Formation of higher fluorides is favored using higher fluorine proportions and higher total pressure. Key reactions outlined.
• All xenon fluorides sublime readily in a vacuum and readily decompose with water, with XeF2 exhibiting the slowest decomposition.
• XeF₄, and XeF₆ decompose rapidly.
• XeF₆, XeF₄, and XeF₂ reacts with other substances in specified reactions.
• [XeF₅]⁻ ion is one of two known pentagonal planar species, the other being [IF₅]²⁻.

Structures of Xenon Halides (VSEPR Model)

• Xenon halide structures cannot be explained by conventional valence bond theory.
• This theory assumes octet acquisition by elements, but noble gases already have octet configurations.
• Xenon compound structures are considered hypervalent compounds where valence electron counts are even number greater than 8.
• The vapor state vibrational spectrum of XeF₆ shows an octahedron distorted by an active lone pair in a centered face.
• XeF₆ molecules may theoretically have an octahedral structure (where the lone pair is in the inactive s-orbital) or a distorted octahedral structure (where the lone pair is active).
• In fact, XeF₆ exhibits fluxionality in gas phase. It exchanges between structures where the lone pair passes through an F₃ triangle (one octahedral face) of the distorted octahedral XeF₆ molecule.

Assignment Questions

• Nuclear fission/fusion: Notes required.
• Solubility product/common ion effect: Explanation needed.
• Xenon fluorides' structure: Explanation for why conventional valence bond theory fails is needed.
• MO diagram of XeF₂: 3c-2e bond explanation necessary.