Difference Between Closo Nido and Arachno Boranes

The key difference between closo nido and arachno boranes is that closo structure has n+1 skeletal bonding electron pairs, and nido structure has n+2 skeletal bonding electron pairs, whereas arachno structure has n+3 skeletal bonding electron pairs.

The structure of borane comes in three types as closo, nido and arachno skeletal structures, depending on the arrangement of skeletal atoms (boron atoms). These three types can be categorized according to a set of rules named “wade rules”. The wade rules are a correlation between the number of electrons, the formula and the shape of the borane structure.

CONTENTS

1. Overview and Key Difference
2. What is Closo Borane
3. What is Nido Borane
4. What is Arachno Borane
5. Side by Side Comparison – Closo vs Nido vs Arachno Boranes in Tabular Form
6. Summary

What is Closo Borane

Closo borane is one of the three major structures of borane which has the chemical formula B_nH_n^2-. The boron atoms of these structures are in the corners of a polyhedron. A common example is B₆H₆^2-.

Figure 01: B₆H_6^2- Structure

According to wade rules, the clusters of the borane having “n” number of skeletal atoms have n+1 skeletal bonding electron pairs. But, before applying this rule, we need to know the number of skeletal electron pairs in a cluster of borane. E.g. each BH unit equals two skeletal bonding electrons, and each boron atom gives three skeletal electrons.

What is Nido Borane?

Nido borane is a type of borane structure having the chemical formula B_nH_n+4. For example, the borane B₅H₉ has the Nido borane structure. This borane structure has the geometry of a square pyramid in which one corner is missing.

Figure 02: Shape of B₅H₉

According to the wade rules, the clusters of the borane having “n” number of skeletal atoms (n = number of boron atoms) have n+2 skeletal bonding electron pairs.

What is Arachno Borane?

Arachno borane is a type of borane structure that has the chemical formula B_nH_n+6. For example, borane B₄H₁₀ has the arachno borane structure. This borane structure has the geometry of an octahedron in which two corners are removed.

Figure 03: Structure of B₄H₁₀

According to wade rules, the clusters of the borane having “n” number of skeletal atoms (n = number of boron atoms) have n+3 skeletal bonding electron pairs.

What is the Difference Between Closo Nido and Arachno Boranes?

There are three types of borane structures as closo, nido and arachno borane. These are named as cage structures because they look like cages. The key difference between closo nido and arachno boranes is that closo structure has n+1 skeletal bonding electron pairs, and nido structure has n+2 skeletal bonding electron pairs, whereas arachno structure has n+3 skeletal bonding electron pairs. These structures are named depending on the wade rules.

Furthermore, the shape of closo borane is a polyhedron, while the shape of nido structure is a square pyramid in which one apex is missing. The shape of arachno borane, on the other hand, is an octahedran in which two corners are removed. A common example for closo structure is B₆H₆^2- ; for nido structure, an example is B₅H₉; for arachno structure, an example is B₄H₁₀.

Below tabulation summarizes the difference between closo nido and arachno boranes.

Summary – Closo vs Nido vs Arachno Boranes

There are three types of borane structures as closo, nido and arachno borane. They are named as cage structures because they look like cages. The key difference between closo nido and arachno boranes is that closo structure has n+1 skeletal bonding electron pairs, and nido structure has n+2 skeletal bonding electron pairs, whereas arachno structure has n+3 skeletal bonding electron pairs.

Reference:

1. “24.5: The Isolobal Principle and Application of Wade’s Rules.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.
2. “Boranes- Assigning Closo, Nido, Arachno, Hypo Nomenclature.” All Bout Chemistry, Noorul Huda, 1 Jan. 1970, Available here.
3. “Boranes.” Wikipedia, Wikimedia Foundation, 21 Dec. 2019, Available here.

Image Courtesy:

1. “B6H6 otahedron” By Episodesn – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Pentaborane-3D-balls” (Public Domain) via Commons Wikimedia
3. “Tetraborane-3D-balls” By Ben Mills – Own work (Public Domain) via Commons Wikimedia

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