Anion⋅⋅⋅Anion Attraction in Complexes of MCl₃^- (M=Zn, Cd, Hg) with CN^.

High-level ab initio calculations show that the MCl₃^- anions comprising Group 2B M atoms Zn, Cd, and Hg form a stable complex with the CN^- anion, despite the like charge of the two ions. The complexation occurs despite a negative π-hole region above the M atom of MCl₃^- . The dimerization distorts the planar geometry of MCl₃^- into a pyramidal shape which reduces the negative potential above the M atom, facilitating a close approach of the two anions, with R(M⋅⋅⋅C)∼2 Å, and an overall attractive electrostatic attraction within the dimer. In the gas phase, this dimer is less stable than the pair of separated ions by some 30 kcal/mol. However, the dissociation must surmount an energy barrier of roughly 25 kcal/mol which occurs at an intermolecular distance of 4 Å. In aqueous solution, the dimerization process is exothermic and barrier-free, with a binding energy in the 11-18 kcal/mol range.