Sigma-hole cooperativity in anionic [FX⋯CH3⋯YF]− (X, Y = Cl, Br) complexes

A computational study of anionic complexes of general formula [FX⋯CH3⋯YF]− (X, Y = Cl, Br) predicted minimum energy structures with unusual bonding features. The two F atoms, the halogens X and Y and the central C atom all lie on the same straight line, with the three H atoms forming covalent CH bonds perpendicular to this straight line. The bonding analysis suggests that the extra negative charge is localized on the central C atom and interacts favourably with the sigma-holes originating from the adjacent FX and YF molecular subunits. This unusual cooperative bonding results in energetically stable complexes.