# Can an interaction be considered both hard-hard and soft-soft?

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?

References

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.