Polyhedral Skeletal Electron Pair Theory - Wade-Mingo’s Rule:
Introduction:
S This is a useful tool to classify the polyhedral Borane clusters.
S It can be conveniently done on the basis of valence electrons.
S This theory originated by K. Wade elaborated by Mingos.
S The type of structure adopted by the particular compound is related to no of electrons available for bonding within the framework.
S 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 the structural relationship of various boranes.
There are several tricks and formulas to find the polyhedral structure of borane compounds. Here I am discussing one which is simple and effective for Boranes and Carboranes.
Wade’s Formula for Boranes and Carboranes:
The no. of framework electrons can be calculated by the following equation:
Boranes: "F=3B+H+Z-2n"
Carboranes: "F=3B+4C+H+Z-2n"
B = No. of Boron atoms; H = No. of hydrogen atoms; C = No. of Carbon atoms
n = no. of Boron atoms (Boranes) / no. of Boron + Carbon atoms (carboranes)
Z = No of charge on the atoms (if positive (minus it) or if negative (add it))
i. If F "3B+H+Z−2n" = 2n + 0; Then they are Capped Closo Boranes.
ii. If F "3B+H+Z−2n" = 2n + 2; Then they are Closo Boranes.
iii. If F "3B+H+Z−2n" = 2n + 4; Then they are Nido Boranes.
iv. If F "3B+H+Z−2n" = 2n + 6; Then they are Arachano Boranes.
v. If F "3B+H+Z−2n" = 2n + 8; Then they are Clado Boranes.
vi. If F "3B+H+Z−2n" = 2n + 10; Then they are Hypo Boranes.
Example :
B6H62-
Soln:
Boranes: F = 3B+H+Z+2n
B = 6; H = 6; Z = 2; 2n=12
F = 3(6)+6+2-2(6) = 14
Then 12+2 = 2n+2
closo Boranes
E
