Charge Distributions and Edge Bond Orders

Calculated Charge Distributions and Edge Bond Orders 

Collectively, these systems show how a hypercarbon atom occupying a pyramidal site above a ring of three, four, or five boron atoms (themselves occupying equatorial sites when n = 5,6, or 7, or tropical sites when n = 10 or 12) forms C-B bonds of increasing length and decreasing bond order as n or k increases. The data also show how the charges on the CH and BH units vary with n and k, and the extent to which the skeletal electron distribution in these systems varies as one or two BH units in the parent cluster are replaced by CH units. 

The relative electronegativities of carbon, boron, and hydrogen (2.5, 2.0, and 2.1 on the Pauling scale, respectively) require hydrogen atoms to bear positive charges (ca. -1-0.25) when attached to carbon, but negative (ca. -0.05) when attached to boron, and boron-carbon bonds to be polar B(8-i-)-C(8-). 

Interestingly, despite the greater electron affinity of carbon than boron, the calculated bond orders/indices of the cage bonds from the equatorial or tropical boron atoms to the axial atoms in systems [B H ] , [CB iH ], and C2B 2H change little as axial carbon atoms replace axial boron atoms. However, axial carbon atoms reduce the electron density and so the bond order/index between equatorial or tropical boron atoms,  

In contrast to the limitations noted above in the value of localized bond treatments for closo carboranes, MO treatments and computations based thereon provide really useful insight into their bonding. An early illustration of their value for closo deltahedral borane dianions B 11 was provided in 1955 by 

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