Nature of Alkali‐ and Coinage‐Metal Bonds versus Hydrogen Bonds

Abstract

We have quantum chemically studied the structure and nature of alkali‐ and coinage‐metal bonds (M‐bonds) versus that of hydrogen bonds between A−M and B− in archetypal [A−M⋅⋅⋅B]− model systems (A, B=F, Cl and M=H, Li, Na, Cu, Ag, Au), using relativistic density functional theory at ZORA‐BP86‐D3/TZ2P. We find that coinage‐metal bonds are stronger than alkali‐metal bonds which are stronger than the corresponding hydrogen bonds. Our main purpose is to understand how and why the structure, stability and nature of such bonds are affected if the monovalent central atom H of hydrogen bonds is replaced by an isoelectronic alkali‐ or coinage‐metal atom. To this end, we have analyzed the bonds between A−M and B− using the activation strain model, quantitative Kohn‐Sham molecular orbital (MO) theory, energy decomposition analysis (EDA), and Voronoi deformation density (VDD) analysis of the charge distribution.

The pnictogen bond: a quantitative molecular orbital picture

“…Our analyses highlight that pnictogen bonds share strong similarities with the corresponding chalcogen bonds (ChB), halogen bonds (XB), and hydrogen bonds (HB). 8 We find that these bonds have considerable covalency on top of electrostatic attraction and can range in strength roughly between −3 and −78 kcal mol −1 (see Fig. 7 ).…”

“… 17 Again, this is equivalent to what was found for chalcogen bonds D 2 Ch⋯A − , halogen bonds DX⋯A − , and hydrogen bonds DH⋯A − . 8 As aforementioned, pnictogen bonds have both an electrostatic component (Δ V elstat ) and a covalent component (Δ E oi ) stemming mainly from the HOMO–LUMO interaction between the occupied halide n p atomic orbital (AO) and the σ* D–Pn antibonding 5a′ acceptor orbital, shown schematically in Fig. 2 .…”

“…These features are also observed for chalcogen bonds D 2 Ch⋯A − , halogen bonds DX⋯A − , and hydrogen bonds DH⋯A − , which makes them similar to pnictogen bonds. 8 …”

“…The electron-donating capacity of the halides is reduced as the halide n p AOs become more diffuse and lower in energy from A − = F − to Br − , thus weakening Δ E oi . 8,17 b This will also result in longer bonds and weaker electrostatic attraction. As a result, the interaction energy (Δ E int ) and, thus, the net pnictogen-bond strength Δ E become less stabilizing along A − = F − to Br − (see Table 1 and Table S1 in the ESI † ).…”

“…From these data, we have constructed a unified framework to rationalize the nature of pnictogen bonds, chalcogen bonds, halogen bonds, and hydrogen bonds, by studying the associated electronic structure and bonding mechanism. 8 …”