Probing covalency in halogen bonds through donor K-edge X-ray absorption spectroscopy: polyhalides as coordination complexes.

The properties of halogen bonds (XBs) in solid-state IX and IX materials (where X = Cl, Br) are explored using donor K-edge X-ray absorption spectroscopy (XAS) to experimentally determine the degree of charge transfer in such XB interactions. The degree of covalency in these bonds is substantial, even in cases where significantly weaker secondary interactions are observed. These data, in concert with previous work in this area, suggests that certain halogen bonds have covalent contributions to bonding that are similar to, and even exceed, those observed in transition metal coordinate bonds.