Chameleon-like Nature of Anagostic Interactions and Its Impact on Metalloaromaticity in Square-Planar Nickel Complexes

Abstract

Anagostic C−H•••M (M = a metal center) intramolecular interactions, one of the most fundamental and elusive forces in organometallic chemistry, are intuitively considered as repulsive and purely electrostatic in nature because of significant metal−hydrogen distances (∼2.3−3.0 Å). Contrary to the current state of knowledge, it is shown herein by quantum chemical computations based on the case study of new square-planar Ni II isomers based on Nthiophosphorylated thiourea that despite significant metal− hydrogen anagostic distances, the covalent-type charge delocalization contribution [Ni(d z 2 ) → σ*(C−H) and σ(C− H) → Ni(d z 2)] exists and it covers, together with the London dispersion energy, up to ∼40% of the overall anagostic stabilization. This charge delocalization component is found to amplify the metalloaromaticity phenomenon although a lack of any stabilizing charge transfer is expected at such long metalhydrogen distances (>3 Å). Remarkably, for the relatively short regime (<3 Å) of anagostic distances, the electrostatic Coulomb forces are destabilizing, which leads to the repulsive anagostic interactions, whereas, surprisingly, an increase of anagostic distance above 3 Å makes anagostic interactions stabilizing mostly because of attractive Coulomb forces. It shows unprecedented agostic (attractive) ↔ anagostic (repulsive) transitions in ubiquitous d 8 square-planar Ni II complexes containing elongated metal−hydrogen distances.

Formation of active species from ruthenium alkylidene catalysts—an insight from computational perspective

“…Particularly, ΔE dispersion is extremely significant at absolute level; it is −141 kJ mol −1 for 2 and − 144 kJ mol −1 for 1 ( Table 2). It is in accord with the recent findings which highlight that bulky substituents might lead to enhancement of the molecular stability since their London dispersion donating properties might easily outweigh the corresponding steric (Pauli/kinetic) repulsion [59][60][61][62][63][64]. In the forthcoming paragraphs, electron density reorganization channels due to formation of various types of non-covalent interactions are identified and discussed in detail.…”

“…The former involves charge transfer from the lone pair of chlorine to the empty σ*(C-H), whereas the latter consist of donation from σ(C-H) to the empty π* of phenyl ring. Additionally, homopolar dihydrogen close contacts CH … HC lead to the two ways charge delocalization channels σ(C-H)➔ σ*(C-H) (and vice-versa) [61][62][63][64]. They all amount to ΔE orb (4 + 5 + 6) = −14 kJ mol −1 (Fig.…”

Structural versatility of the quasi-aromatic Möbius type zinc(ii)-pseudohalide complexes – experimental and theoretical investigations

“…In 2, the cation is a doubly charged centrosymmetric dinuclear structure with two Zn II [57][58][59][60][61] study, that the seven-membered rings in 1-3 constitute a quasi-aromatic Möbius-type motif, though the absolute magnitude of such p-delocalization is notably weaker than in the corresponding cadmium-based analogs. 24,27 Bulkiness of the ligands (L) are found not only to amplify London dispersion stabilization, 24,27,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] but also inuence the magnitude of quasi-p-delocalization (of Möbius-type) through modi-cation of the polarity of Zn-L bonding.…”

“…The prevalence of the DE disp term is consistent with recent ndings, which rediscover the importance of London dispersion forces in small and sizeable species. 24,27,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] In 3 the [Zn(NCS) 4 ] 2À anion sticks very strongly (DE int ¼ À193.21 kcal mol À1 ) to two neighboring stacked [Zn(NCS)L II ] + units primarily through electrostatic forces (75.3% of the overall stabilization) (Fig. 5).…”

“…7). It demonstrates the two-folded role of bulky substituents: they are not only dispersion donors, [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] but they also lead to amplication of the 7-MR twist (and enhanced quasiaromaticity). Previously only the former feature has been recognized.…”