The Weakness of B−H⋅⋅⋅π Interactions: Much Softer than a Hydrogen Bond

Abstract

Recently, a new type of non-covalent interaction, the B-H⋅⋅⋅π interaction, was introduced. Nevertheless, the unique determination of individual, non-covalent bond strengths in complex molecular systems is a non-trivial task with often controversial results. The utilization of generalized compliance constants (relaxed force constants) is proposed as unique bond-strength descriptors for classical and non-classical hydrogen bonds. The strength of recently proposed B-H⋅⋅⋅π bonds is discussed.

B–H⋯π: a nonclassical hydrogen bond or dispersion contact?

“…3 However, the strength of these interactions has very recently been questioned. 4 Moreover, such an interpretation of the crystallographic data contradicts the wealth of evidence for unique noncovalent interactions of boron hydrides. 5 The aim of this study is thus to gain deeper insight and to reveal the nature of the noncovalent interactions in the B-HÁ Á Áp motif.…”

“…Let us note that the amphiphilicity of the BH groups of carborane will be influenced by their position on the cage, i.e. growing in the order BH (9, 12) o BH (8, 10) o BH (4,5,7,11) o BH (3,6).…”

Ill‐defined chemical concepts: The problem of quantification

“…Figure , reproduced with permission from Ref. tells another story: First of all, force constants (lower curve, Figure ) mirror the variation of the force between atoms in a molecule, not the force itself. In the case of an energy minimized structure, all atom–atom force constants, bonded in a Lewis sense or not are ≠ 0.…”

“…It his somehow disturbing that those dilemmas often just pop up when it comes to the interplay between computational chemistry and experiment. Particular the quantification of concepts like aromaticity or bond order and the characterization of weak interactions seem to be notorious cases . With this perspective, I would like to point out that some of the contemporary publications in chemistry are indeed characterized by a confusion of ideas and concepts.…”

Nature and Strength of the Inner‐Core H⋅⋅⋅H Interactions in Porphyrinoids

“…The π electrons can be potential H‐bond acceptors to form S−H ⋅⋅⋅ π, C−H ⋅⋅⋅ π, N−H ⋅⋅⋅ π, and O−H ⋅⋅⋅ π H‐bonds. Very recently, Zhang and co‐workers observed B−H ⋅⋅⋅ π nonclassical H‐bonds, which emphasizes the fact that π electrons can be involved in H‐bond formation for X δ − −H δ + donors and also for X δ + −H δ − donors . The concept of the antielectrostatic H‐bond has also been highlighted, as elegantly explained by Weinhold and co‐workers for the H‐bonds between fluoride (F − ) and bicarbonate (HCO 3 − ).…”