Understanding the Behavior of Halogens as Hydrogen Bond Acceptors

The similarities and differences between the behavior of carbon-bound and terminal metal-bound halogens and halide ions as potential hydrogen bond acceptors has been extensively investigated through examination of many thousands of interactions present in crystal structures. Halogens in each of these environments are found to engage in hydrogen bonding, and geometric preferences for these interactions have been established. Notably, typical H···X−M angles are markedly different for X = F than for X = Cl, Br, I. Furthermore, there are significant parallels between the behavior of moderately strong hydrogen bond acceptors X−M and the much weaker acceptors X−C. The underlying reasons for the observed geometric preferences have been established by ab initio molecular orbital calculations using suitable model systems. The results are presented within the context of their potential applications in crystal engineering and supramolecular chemistry, including relevance to nucleation in halogenated solvents. The broader implications of the results in areas such as halocarbon coordination chemistry, binary metal halide solid-state chemistry, and the study of weakly coordinating anions are also discussed.