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 BnHn2-. The boron atoms of these structures are in the corners of a polyhedron. A common example is B6H62-.
Figure 01: B6H62- 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 BnHn+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 BnHn+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 B6H62- ; 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