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1. Anion–anion and anion–neutral triel bonds

   https://doi.org/10.1039/D0CP06547A  

   Wysokiński, Rafał; Michalczyk, Mariusz; Zierkiewicz, Wiktor; Scheiner, Steve (March 2021, Physical Chemistry Chemical Physics)

   null (Ed.)

   The ability of a TrCl 4 − anion (Tr = Al, Ga, In, Tl) to engage in a triel bond with both a neutral NH 3 and CN − anion is assessed by ab initio quantum calculations in both the gas phase and in aqueous medium. Despite the absence of a positive σ or π-hole on the Lewis acid, strong triel bonds can be formed with either base. The complexation involves an internal restructuring of the tetrahedral TrCl 4 − monomer into a trigonal bipyramid shape, where the base can occupy either an axial or equatorial position. Although this rearrangement requires a substantial investment of energy, it aids the complexation by imparting a much more positive MEP to the site that is to be occupied by the base. Complexation with the neutral base is exothermic in the gas phase and even more so in water where interaction energies can exceed 30 kcal mol −1 . Despite the long-range coulombic repulsion between any pair of anions, CN − can also engage in a strong triel bond with TrCl 4 − . In the gas phase, complexation is endothermic, but dissociation of the metastable dimer is obstructed by an energy barrier. The situation is entirely different in solution, with large negative interaction energies of as much as −50 kcal mol −1 . The complexation remains an exothermic process even after the large monomer deformation energy is factored in. 

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2. Crystallographic and Theoretical Evidences of Anion⋅⋅⋅Anion Interaction

   https://doi.org/10.1002/cphc.202100132  

   Wysokiński, Rafał; Zierkiewicz, Wiktor; Michalczyk, Mariusz; Scheiner, Steve (April 2021, ChemPhysChem)

   Abstract Planar (HgCl₃)⁻anions are stacked fairly closely together in a slipped parallel arrangement within several crystal structures. Quantum chemical analysis shows evidence of strong noncovalent spodium bonds between the Hgπ‐hole of one unit and the Cl atom of an adjacent unit. Anion⋅⋅⋅anion spodium bonds work in tandem with crystal packing forces. 

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3. Anion–Anion Interactions in Aerogen-Bonded Complexes. Influence of Solvent Environment

   https://doi.org/10.3390/molecules26082116  

   Grabarz, Anna; Michalczyk, Mariusz; Zierkiewicz, Wiktor; Scheiner, Steve (April 2021, Molecules)

   null (Ed.)

   Ab initio calculations are applied to the question as to whether a AeX5− anion (Ae = Kr, Xe) can engage in a stable complex with another anion: F−, Cl−, or CN−. The latter approaches the central Ae atom from above the molecular plane, along its C5 axis. While the electrostatic repulsion between the two anions prevents their association in the gas phase, immersion of the system in a polar medium allows dimerization to proceed. The aerogen bond is a weak one, with binding energies less than 2 kcal/mol, even in highly polar aqueous solvent. The complexes are metastable in the less polar solvents THF and DMF, with dissociation opposed by a small energy barrier. 

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4. Triel bonds within anion ··· anion complexes

   https://doi.org/10.1039/D1CP04296C  

   Michalczyk, Mariusz; Zierkiewicz, Wiktor; Wysokiński, Rafał; Scheiner, Steve (November 2021, Physical Chemistry Chemical Physics)

   The ability of two anions to interact with one another is tested in the context of pairs of TrX 4 − homodimers, where Tr represents any of the triel atoms B, Al, Ga, In, or Tl, and X refers to a halogen substituent F, Cl, or Br. None of these pairs engage in a stable complex in the gas phase, but the situation reverses in water where the two monomers are held together by Tr⋯X triel bonds, complemented by stabilizing interactions between X atoms. Some of these bonds are quite strong, notably those involving TrF 4 − , with interaction energies surpassing 30 kcal mol −1 . Others are very much weaker, with scarcely exothermic binding energies. The highly repulsive electrostatic interactions are counteracted by large polarization energies. 

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5. Probing Anion–Molecule Complexes of Atmospheric Relevance Using Anion Photoelectron Detachment Spectroscopy

   https://doi.org/10.1021/acsphyschemau.2c00060  

   Jarrold, Caroline Chick (December 2022, ACS Physical Chemistry Au)