Astatine Facing Janus: Halogen Bonding vs. Charge-Shift Bonding

Abstract

The nature of halogen-bond interactions was scrutinized from the perspective of astatine, potentially the strongest halogen-bond donor atom. In addition to its remarkable electronic properties (e.g., its higher aromaticity compared to benzene), C6At6 can be involved as a halogen-bond donor and acceptor. Two-component relativistic calculations and quantum chemical topology analyses were performed on C6At6 and its complexes as well as on their iodinated analogues for comparative purposes. The relativistic spin–orbit interaction was used as a tool to disclose the bonding patterns and the mechanisms that contribute to halogen-bond interactions. Despite the stronger polarizability of astatine, halogen bonds formed by C6At6 can be comparable or weaker than those of C6I6. This unexpected finding comes from the charge-shift bonding character of the C–At bonds. Because charge-shift bonding is connected to the Pauli repulsion between the bonding σ electrons and the σ lone-pair of astatine, it weakens the astatine electrophilicity at its σ-hole (reducing the charge transfer contribution to halogen bonding). These two antinomic characters, charge-shift bonding and halogen bonding, can result in weaker At-mediated interactions than their iodinated counterparts.

Equivalent and Complement of the ω-Bonding Model and Charge-Shift Bonding Model: A Natural Bond Orbital/Natural Resonance Theory/Atoms in Molecules Investigation via Cu/Ag/Au Bonding in BMX (B = H2O, H2S, NH3, and PH3; M = Cu, Ag, and Au; and X = F, Cl, Br, and I)

“…Nevertheless, with replacement of Cu/Ag/Au in BMX with halogen atoms, one obtains the halogen-bonded complexes. On the basis of the present studies and the recent studies on the interplay between halogen bonding and CS bonding, , it is anticipated that many halogen-bonded complexes will also exhibit a CS bonding nature. Following this study, further NBO/NRT/AIM studies are underway to confirm this anticipation.…”

“…However, we anticipate that CS bonding should also be important in understanding Cu/Ag/Au bonding based on some studies by Shaik et al. − The CS bond belongs to a new class of chemical bonds, emerging from the valent bond (VB) theory, and it is supported by the atoms in molecules (AIM) theory. − Since 1992, this model has been successfully applied to understand the bonding in diatomic halogen molecules (e.g., F 2 ), hypermolecules (e.g., Cl 3 – and XeF 2 ), , transition metal species (e.g., Au 2 and MX), , etc. Our recent study on xenon hydrides using NBO/NRT analyses demonstrated that the H–Xe bonds in all studied molecules exhibit a CS bonding character .…”

“…For example, in the report by Silvi et al, the DI of the F–F bond (in F 2 ) is 1.24 and that of the B–N bond (in BH 3 NH 3 ) is 0.38. Recent reports by Galland et al , have demonstrated the practical applications of DI in explaining the CS bonding nature in At 3 C–At···Cl – and C 6 At 6 species. In our calculations, the DI values for M–X bonds in free MX are listed in the last column of Table .…”

Charge-Shift Bonding Propensity in Halogen-Bonded BXY (B Is a Small Lewis Base H2O or NH3; X and Y Are Halogen Atoms) Complexes: An NBO/NRT/AIM Investigation

“…Another example is that the Cu/Ag/Au bonds (also known as coinage-metal bonds) [15] in BMY (B = H 2 O, H 2 S, NH 3 , and PH 3 ; M = Cu, Ag, and Au; and Y = F, Cl, Br, and I) complexes have been established to show the characters of both CS bonding and ω-bonding in our recent work [16] by using the NBO/NRT/AIM methods. Additional examples of CS bonding could be found in recent publications [17][18][19][20]. Here, it is necessary to introduce the ω-bonding model because the studied halogen bonding, as shown in the following, exhibits both CS bonding and ω-bonding characters.…”

The Pnictogen Bond: The Covalently Bound Arsenic Atom in Molecular Entities in Crystals as a Pnictogen Bond Donor

“…There are relatively few reviews [ 3 , 18 , 19 , 20 , 21 , 22 ], original papers (for example [ 1 , 5 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ]), overviews [ 30 ], and comments on the strength [ 31 ], nature [ 32 , 33 ], and propensity of elements of the pnictogen family (Group 15) to engage in pnictogen bonding [ 34 , 35 , 36 ]. The reason for this is probably because recent work on noncovalent interactions has focused largely on exploring σ-hole and π-hole interactions [ 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 ] associated with the main elements of the periodic table, with particular attention to the elements of Groups 14, 16, and 17, including hydrogen bonding [ 49 ].…”