Halogen bonding and beyond: factors influencing the nature of CN-R and SiN-R complexes with F-Cl and Cl2

It was recently shown that complexes of the type F-Cl center dot center dot center dot CN-R, ostensibly halogen bonded, sometimes have properties (relatively high binding energies, short Cl-C separations and considerably lengthened F-Cl distances) that are inconsistent with typical halogen bonds (Del Bene et al. in J Phys Chem A 114: 12958-12962, 2010). We attribute these anomalous features, as well as analogous observations for F-Cl center dot center dot center dot SiN-R systems, to the strong polarization of the CN-R carbons and SiN-R silicons by the electric field of the positive sigma-hole of the F-Cl chlorine. This polarization may evolve into some degree of dative sharing of electrons by the carbons and silicons. This interpretation is supported by the fact that complexes of CN-R and SiN-R with Cl-Cl, which has a much weaker positive sigma-hole than F-Cl, are considerably less likely to show the unusual features. It is demonstrated that the full ranges of binding energies of the CN-R and SiN-R complexes with either F-Cl or Cl-Cl can be represented well (R-2 > 0.96) in terms of the most negative electrostatic potentials and the lowest local ionization energies on the carbon and silicon surfaces. These properties reflect the electrostatic components of the interactions and the polarizabilities/dative reactivities of the carbons and silicons.