Delocalized Bonding in Li₂X₂Rings: Probing the Limits of the Covalent and Ionic Bonding Models

Abstract

In spite of the highly ionic character of most lithium-element bonds, the bonding within Li2X2 rings presents similarities with that found in analogous transition metal systems. They obey simple framework electron counting rules that allow us to predict whether they will form a regular ring or a squeezed one with short Li–Li or X–X distances. A combined computational and structural database analysis discloses the orbital conditions that determine the framework electron counting rules. These systems probe the borderline between the covalent and ionic bonding models since, paradoxically, a non-negligible covalent contribution of the two Li atoms (formally Li+ ions in the ionic model) to the Li–X framework bonding favors them approaching each other within bonding distance.

Halogen Bonds of Halogen(I) Ions─Where Are We and Where to Go?

“…Halogen(I) complexes do not behave as coordinative transition metal complexes, 43 for example, when exposed to solvents and counterions. 44 The bond has occasionally also been named a halonium bond 45 and the complexes superhalides. 21,22 We note that halogen(I) com-plexes should not be misconceived with λ 3 -halogane (λ 3iodane, λ 3 -bromane, λ 3 -chlorane) compounds, which sometimes are also termed halonium complexes yet encompass a halogen(III).…”

“…13,19 Still, a simple electrostatic model is unable to explain the stability of three-center halogenbond complexes. 45 These halogen bonds are characteristically short and strong, ΔG > 50 kJ/mol, with their bond strength and length depending on the involved halogen and Lewis bases. The bond distance is often characterized by the reduction of the contact distance of the interacting atoms in comparison to the sum of their van der Waals radii, which for three-center bonds is 60−70% (R XB = 0.6−0.7).…”

“…13,26 The bond lengths and the bond strengths decrease with the size of the involved halogen(I) decreasing. For [bis(pyridine)halogen- 19,45 Initially, even fluorine(I) complexes have been predicted to be symmetric, 70 a suggestion that was later experimentally and computationally refuted. 19,41 The bond energies strongly depend on the Lewis basicity of the electron donors involved.…”

“…82 This observation is also in line with the strength independence of the N−I bond lengths of [bis(pyridine)iodine(I)] + complexes, 62 56 The influence of substituents on the stability and the geometry of halogen(I) complexes has been extensively studied using solution NMR, X-ray diffraction, and computations. 45,62,63,80 In short, electron-donating substituents strengthen while electron-withdrawing ones weaken the 3c4e halogen bond. It is important to note that the halogen-bond strength shows a strong dependence on the electron density of the Lewis base, whereas the halogen-bond donor−acceptor distance and the NMR coordination shifts do not show a larger strength-dependent variation.…”

From polygons to polyhedra through intermediate structures. A shape measures study of six-atom inorganic rings and clusters

“…The lithium atoms in many compounds, though formally present as cations, have a marked tendency to group together forming clusters of varying sizes. A recent study 104 analysed the role of Lewis bases coordinated to lithium in favouring the proximity of lithium ions in the case of dilithium groups. It is therefore timely to look for stereochemical trends among lithium clusters, as presented in this section for Li 6 cores.…”