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The chemical bond is the biggest paradigm in chemistry. But at the same time it is notoriously complex to define it. Under the assumption that knowing what we do not know makes better students and lecturers, we succinctly describe three approaches to define the bond (energetic, structural, and orbital), their advantages and especially their failures. We propose that these definitions, including their theoretical, practical and even philosophical issues, should be taught to advanced undergraduate chemistry students as an essential introduction to the chemical bond module of quantum chemistry courses. This is a controversial topic due to the lack of consensus in the chemical bond community over this heavily disputed topic and the conflicting pedagogical approach; however, and maybe because of this controversy, the teaching of the definitions of the chemical bond including their open questions and challenges can be positively instructive. Moreover, we propose that teaching the uncertainties of chemistry, especially in relation to the paradigmatic chemical bond, is an ethical matter.

Heavy atom quantum tunnelling was already observed in several π‐bond‐shifting reactions of antiaromatic molecules. In these, their Jahn‐Teller distortions create sharp energy profiles with small atomic trajectories, which are ideal for observing tunnelling. Herein, we studied the effect of the nitrogen in the N‐heterocyclic pentalenes, distinguishing between isothermic and exothermic reactions. The reactivity was not easily predictable, with our computations revealing a wide variability of rates, from the complete cancellation of the barrier to high isomer stability. For all these systems, quantum tunnelling emerged as the most significant mechanism at room temperature and the only valid one at cryogenic conditions. [ABSTRACT FROM AUTHOR]

The role of quantum tunnelling (QT) in the proton transfer kinetics of the uranyl tetrahydroxide (UTH, [UO2(OH)4]2−) cis to trans isomerization was computationally studied under three possible reaction pathways. The first pathway involved a direct proton transfer from the hydroxide ligand to the oxo atom. In the other two pathways, one or two water molecules were added to the second sphere. The first H2O, bound by hydrogen bonds to the ligands, acts as a bridge enabling a proton shuttling, a concerted hopping of a proton from the hydroxide to the oxo atom similar to the Grotthuss mechanism. In the third pathway, the second water molecule does not participate in the H-transfer chain, but works as an anchor for the first water molecule, limiting its movement and therefore enhancing the QT. Since experimentally the reaction occurs in water, the first two pathways (no water or one H2O) serve only as models of the gas phase behaviour, while the third pathway will always be thermodynamically and kinetically preferred. The effects were investigated in the gas phase as well as in a continuum aqueous model, including the H/D Kinetic Isotope Effect (KIE). The results indicate that at very low temperatures, QT is the only mechanism that permits the reaction kinetics, consistent with the large computed KIE. At higher temperatures, thermally activated tunnelling competes with the classical crossing over the potential barrier. [ABSTRACT FROM AUTHOR]

There are many types of coordinating bond types, but an inverse bonding‐backbonding (IBBB) with a metal to ligand σ donation and a ligand to metal π backdonation (a "σZ‐πL" interaction) is unheard of. Herein we computationally describe one such case based on a borabenzene ligand and a bismuth pincer complex. We believe that IBBB involving d orbitals in transition metals is impossible, but a π charge transfer to a post‐transition metal empty p orbital can do the trick. [ABSTRACT FROM AUTHOR]