Question

Suggest a structure for the mixed aluminum-boron hydride \(\mathrm{AlBH}_{6}\),

Short answer

Aluminum likely forms bridges with boron and hydrogen, creating a stable complex.

Step-by-step solution

Understand the Compound

The compound given is aluminum-boron hydride, represented as \( \mathrm{AlBH}_6 \). This indicates that the structure involves aluminum (Al), boron (B), and hydrogen (H) atoms. Our goal is to align these atoms in a feasible chemical structure.

Consider Known Structures

Consider how aluminum and boron typically form compounds. Both elements are known to form hydrides. The common boron hydride is \( \mathrm{B_2H_6} \), with a three-center two-electron bond structure (bridge bonding), while Al can form compounds like \( \mathrm{AlH_3} \). Bridges and coordination must be considered.

Allocate Hydrogen Atoms

Assign the six hydrogen atoms in a way that accommodates their bonding capacity and the octet rule for boron and aluminum. Boron typically forms three bonds, while aluminum, in hydride form, can expand its shell by forming coordinate bonds.

Draft a Tentative Structure

Propose a structure where aluminum is coordinated near the center. Consider bridging hydrogen atoms from aluminum to boron akin to how diborane is structured, given the electron-deficient nature of these elements.

Optimize the Proposed Structure

Ensure the structure is electronically stable. A feasible approach is to have a central \(\mathrm{AlH_3}\) unit with additional \(\mathrm{BH_3}\) forming bridges. For example, each boron may satisfy its trivalent nature, perhaps constructing a \(\mathrm{B(\mu-H)_2Al}\) bridge.

Confirm Structural Feasibility

Review the structure ensuring it follows basic rules of bonding and hybridization. Align the total electron count and ensure no atoms have unfilled orbitals unless necessary for specific resonance.

Key concepts

Chemical Bonding

In understanding the chemical bonding of aluminum-boron hydride (\( \mathrm{AlBH}_6 \)), it's essential to explore how atoms bond to form this compound. Aluminum and boron both participate in forming bonds with hydrogen atoms. Aluminum typically forms three bonds, however, it can expand its bonding through the use of d-orbitals for bonding purposes. Boron, being another electron-deficient element, also forms three standard covalent bonds with hydrogen.
To create a stable structure for \( \mathrm{AlBH}_6 \), both bridging and terminal hydrogen atoms are involved. Bridging hydrogen atoms are particularly crucial as they connect two metal atoms, a unique characteristic seen in hydrides involving boron. In this type of bonding, known as three-center two-electron bonds, each hydrogen atom shares only two electrons between aluminum and boron, which is less common compared to typical two-center two-electron covalent bonds.
This type of bonding is significant in enabling the stability of \( \mathrm{AlBH}_6 \), despite the electron deficiency seen in typical aluminum and boron compounds. Understanding these principles is key to grasping the unique nature and structure of aluminum-boron hydride.

Molecular Geometry

The molecular geometry of \( \mathrm{AlBH}_6 \) is influenced by the hybridization and the electron arrangements around the aluminum and boron atoms. Aluminum in \( \mathrm{AlBH}_6 \) can adopt a trigonal planar or tetrahedral arrangement due to its ability to bypass the octet rule and accommodate more electrons. Boron tends to favor a trigonal planar arrangement. The overall geometry results from two influences:

• The ability of aluminum to draw bonds from hydrogen due to its flexibility in expanding its valence shell.
• The role of bridging hydrogens which help stabilize the electron-deficient centers and allow effective sharing of electrons between boron and aluminum.

This unique placement of atoms contributes to the overall structure and stability of the compound. Bridging atoms play a crucial role, meaning that the molecule may not be simply described by traditional VSEPR theory as electron sharing deviates from the strict sole-terminal bonding seen in other hydrides.

Electron-Deficient Compounds

Electron-deficient compounds are those where atoms do not fully adhere to the traditional octet rule. \( \mathrm{AlBH}_6 \) is a classic example of this because neither aluminum nor boron achieves a complete octet in its bonding. Yet they manage stability through unique bonding arrangements. In this compound, bridging hydrogens facilitate what is known as "electronic sharing." This allows each adjacent boron and aluminum to maintain some level of stability despite having fewer than eight electrons in their valence shell.

• Bridge bonds serves to distribute electrons across more than two atoms.
• Aluminum's and boron's ability to engage in multi-center bonding enables their structures to be more dynamic but still stable.

Understanding electron-deficient compounds like \( \mathrm{AlBH}_6 \) offers insight into how chemistry can adapt through non-traditional pathways. Such structures challenge classical bonding theories and illustrate new ways atoms stabilize in electron-limited environments.