Halogen vs. ionic bonding: an unusual isomorphism between the neutral (C5Me5)2Fe/C2I2 cocrystal and ionic [(C5Me5)2Fe]Br3 crystal

Almost equal molecular sizes of diiodoacetylene (C₂I₂) and the [Br₃]⁻ anion result in isomorphic neutral cocrystal (DMFc–C₂I₂) and [DMFc]⁺Br₃⁻ salt. This isomorphic pair of neutral and ionic crystals provides a unique example for the analysis of different types of bonding resulting in isotypic solid-state structures. Their energy frameworks visualize the obvious difference between their energetic structures: the lattice of DMFc–C₂I₂ is built upon the chains stabilized by I···(η₅-C₅Me₅) halogen bonds; at the same time, [DMFc]⁺Br₃⁻ consists of electrostatically assembled layers associated by Br···π(Cp*) XBs interactions.