The $\ce {B-F}$ bond can be described as having both a large ionic and a large covalent character$\ce {^1}$, with the AIM charges on $\ce {F}$ and $\ce {B}$ being $\ce {-0.81}$ and $\ce {+2.43}$, respectively, and the bond critical point density being only slightly smaller than that for the $\ce {C-C}$ bond. In a similar vein of thought, can interactions between Lewis acids and Lewis bases be both rather hard and soft at the same time?

The Klopman-Salem equation can be used to quantify the degree of hardness and softness in a Lewis acid-base interaction$\ce {^2}$. The second term is a Coulombic attraction term and the third term is a HOMO-LUMO interaction term. Are there cases where both the 2nd and 3rd terms are large in magnitude?


  1. Matta, C. F.; Gillespie, R. J. Understanding and Interpreting Molecular Electron Density. J. Chem. Educ. 2002, 79(9), 1141 - 1152
  2. Fleming, I. Molecular Orbitals and Organic Chemical Reactions. John Wiley & Sons, Ltd. United Kingdom, 2009.

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