The role of halogen interactions in the crystal structure of biscyclopentadienyl dihalides
The purpose of this survey is to examine the crystal packing of bent metallocenes, namely group IV, V and VI biscyclopentadienyl dihalides and dihydrides, first synthesized in the late 1950’s. This analysis is based on the present knowledge on new types of intermolecular contacts such as halogen and C-H…π interactions as well as dihydrogen bonds. Results are also supported by ab initio calculations that provide information on electrostatic charge distributions, in particular the effect of two V-shaped metal-halogen bonds on the molecular electron density distribution. Type I halogen interactions do not appear to be affected by this feature, specifically the geometric parameters and variation of atom polarizability, but the resultant electronic density distribution precludes the presence of Type II halogen bonds. Special attention is given to the competition among hydrogen and halogen interactions, in particular its influence on the nature and geometric orientations of the different supramolecular motifs and, ultimately, on the space groups observed for each crystal.