On the nature of inter-anion coinage bonds

To explore the binding energy profiles and elucidate the bonding nature in counter-intuitive anionMIDLINE HORIZONTAL ELLIPSISanion coinage bonds (CiBs), thirty-one complexes were constructed, and the inter-anion CiBs were studied theoretically. The metastability was evidenced by the characteristic potential wells in six cases, demonstrating that anions [Au(CN)(4)](-), [Ag(CN)(2)](-) and [AuO](-) are appropriate building blocks for CiBs. The kinetic stability was further supported by ab initio molecular dynamics (AIMD) simulations and the analyses based on the local vibrational mode and quantum theory of atoms in molecules (QTAIM) methods. The anionMIDLINE HORIZONTAL ELLIPSISanion CiBs in the dimers of [AuCl4](-) and [Au(CN)(4)](-) previously observed in condensed phases were confirmed to be thoroughly repulsive under vacuum, but turned attractive in the crystal environment which was simulated using the solvation model based on density (SMD). However, the intrinsic strength of the inter-anion bonding is barely variated by the environment, as it is the combination of the inter-anion interaction and the environment effect that stabilizes the anion pairs. The block-localized wavefunction (BLW) method and its corresponding energy decomposition (BLW-ED) approach were further employed aiming at a chemically meaningful explanation for these counterintuitive phenomena. By inspecting the profiles of energy components, we identified the vital distinction between inter-anion CiBs and conventional non-covalent interactions lying in the electrostatic interaction, which variates nonmonotonically in the inter-anion complexes. The electrostatic interaction also dominates the depth of potential wells, which is commonly used to evaluate the kinetic stability, while Pauli exchange repulsion is the most repulsive factor preventing the formation of anion adducts. The importance of the Pauli exchange repulsion was further highlighted by comparing cases with and without metastability, in which the absence of a potential well is solely caused by the enhancement of the Pauli exchange repulsion.

Automated summary

In this study, the binding energy profiles and bonding nature of counter-intuitive anion coinage bonds (CiBs) were explored by constructing thirty-one complexes and theoretically studying the inter-anion CiBs. The metastability and kinetic stability of these CiBs were supported by potential wells and ab initio molecular dynamics simulations. The importance of the electrostatic interaction and the repulsive factor of Pauli exchange repulsion in the formation and stability of anion pairs were highlighted.