Ken Wade developed a method for the prediction of shapes of borane clusters; however, it may be used for a wide range of substituted boranes (such as carboranes) as well as other classes of cluster compounds.
Wade’s rules are used to rationalize the shape of borane clusters by calculating the total number of skeletal electron pairs (SEP) available for cluster bonding. In using Wade’s rules it is key to understand structural relationship of various boranes
The general methodology to be followed when applying Wade’s rules is as follows:
- Determine the total number of valence electrons from the chemical formula, i.e., 3 electrons per B, and 1 electron per H.
- Subtract 2 electrons for each B-H unit (or C-H in a carborane).
- Divide the number of remaining electrons by 2 to get the number of skeletal electron pairs (SEP).
- A cluster with n vertices (i.e., n boron atoms) and n+1 SEP for bonding has a closo structure.
- A cluster with n-1 vertices (i.e., n-1 boron atoms) and n+1 SEP for bonding has a nido structure.
- A cluster with n-2 vertices (i.e., n-2 boron atoms) and n+1 SEP for bonding has an arachno structure.
- A cluster with n-3 vertices (i.e., n-3 boron atoms) and n+1 SEP for bonding has an hypho structure.
- If the number of boron atoms (i.e., n) is larger than n+1 SEP then the extra boron occupies a capping position on a triangular phase.
You will easily be able to answer following questions after watching this video
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