Question

Which of the following “Borane” is arachno structural type?
(A) ${B_{10}}{H_{14}}$
(B) ${B_2}{H_7}^ - $
(C) ${B_7}{H_7}^ - $
(D) ${B_{10}}{H_{18}}$

Answer 1

Hint: Boranes are the compounds which are hydrides of boron. They have a generic formula of ${B_x}{H_y}$. Depending upon the molecular formula, we can identify arachno boranes. They have a general molecular formula ${B_n}{H_{n + 6}}$ or ${B_n}{H_{n + 5}}^ - $.

Complete answer:
Boranes are the compounds which are hydrides of boron. They have a generic formula of ${B_x}{H_y}$. They have a tendency to form multicenter bonds.
- The cluster of boranes can be divided into certain categories depending upon their structure.
- Closo-boranes are the boranes which have a cage-like structure. They have a complete closed polyhedral structure. The general formula of closo boranes is ${B_n}{H_n}^{2 - }$
- Nido-boranes have nest-like structures. They are not closed structures. General formula of nido boranes can be given as ${B_n}{H_{n + 4}}$.
- Arachno boranes are compounds which have a structure like a spider's web. They have open structure. General formula of arachno boranes can be given by ${B_n}{H_{n + 6}}$ or ${B_n}{H_{n + 5}}^ - $ or ${B_n}{H_{n + 4}}^{2 - }$.
- We will compare the general formula with the formula of boranes given in the options to find the answer.
- ${B_{10}}{H_{14}}$ :
It has the molecular formula like ${B_n}{H_{n + 4}}$ . So, we can say that this borane is a nido borane.
- ${B_2}{H_7}^ - $ :
It has a molecular formula like ${B_n}{H_{n + 5}}^ - $. Thus, we can conclude that this borane is arachno borane.
- ${B_{10}}{H_{18}}$
It has a molecular formula like ${B_n}{H_{n + 8}}$. Thus, this borane is hypho borane.
Thus, we can say that ${B_2}{H_7}^ - $ is an arachno borane.

Therefore, the correct answer of the question is (B).

Note:
Remember that main types of boranes include closo, nido, arachno, hypho and conjuncto boranes. The formulas of boranes can be related by subtracting ${H^ + }$ ions from the formula to make the number of B and H equal.