π covalency in the halogen bond

Halogen bonds are a highly directional class of intermolecular interactions widely employed in chemistry and chemical biology. This linear interaction is commonly viewed to be analogous to the hydrogen bond because hydrogen bonding models also intuitively describe the sigma -symmetric component of halogen bonding. The possibility of -covalency in a halogen bond is not contemplated in any known models. Here we present evidence of pi -covalency being operative in halogen bonds formed between chloride and halogenated triphenylamine-based radical cations. We reach this conclusion through computational analysis of chlorine K-edge X-ray absorption spectra recorded on these halogen bonded pairs. In light of this result, we contend that halogen bonding is better described by analogy to metal coordination bonds rather than hydrogen bonds. Our revised description of the halogen bond suggests that these interactions could be employed to influence the electronic properties of conjugated molecules in unique ways. p id=Par urrent models of halogen bonding describe the sigma -symmetric component of this interaction but do not contemplate the possibility of pi -covalency. Here the authors provide experimental and computational evidence of pi -covalency in halogen bonds involving radical cation halogen bond donors.