Question

The cage-type structure adopted by boron hydride, [B5H11] is

• A. closo
• B. nido
• C. hypo
• D. arachno

Hint:

Boron hydrides can adopt different types of cage structures based on the number of vertices involved. Nido structures have 4 vertices.

Answer 1

The Correct Option is D

The question is based on understanding the structure of boron hydrides, specifically [B₅H₁₁]. In chemistry, boron hydrides have characteristic structures that are classified into different types based on their geometry and electron count. The classification is mainly into the following types:

• Closo: Closed polyhedral structures where the number of boron atoms is equal to one plus the number of vertices in the polyhedron (formula: B_nH_n^2-).
• Nido: Open polyhedral structures, resembling a nest, with the formula B_nH_n+4.
• Arachno: Even more open structures than nido, resembling a web with the formula B_nH_n+6.
• Hypo: Much less common and have fewer hydrogens than the nido type.

The correct classification for [B₅H₁₁] can be determined through its formula. By comparing it with the general formulae:

• For n = 5, the arachno type structure should have 11 hydrogen atoms, i.e., B₅H₅₊₆.

Therefore, [B₅H₁₁] fits the pattern for an arachno borane structure.

The options can be eliminated based on the formula matching:

• Closo: Not possible as the number of hydrogen atoms does not fit the formula B₅H₅₊₂=B₅H₇.
• Nido: Not possible as B₅H₉ would be nido, and not enough hydrogens in B₅H₁₁ for nido.
• Hypo: Doesn't fit as it typically has a lesser number of hydrogens than nido.

Thus, the boron hydride [B₅H₁₁] is correctly classified as having an arachno structure.