Strength of the Pnicogen Bond in Complexes Involving Group Va Elements N, P, and As

Abstract

A set of 36 pnicogen homo- and heterodimers, R3E···ER3 and R3E···E′R′3, involving differently substituted group Va elements E = N, P, and As has been investigated at the ωB97X-D/aug-cc-pVTZ level of theory to determine the strength of the pnicogen bond with the help of the local E···E′ stretching force constants k(a). The latter are directly related to the amount of charge transferred from an E donor lone pair orbital to an E′ acceptor σ* orbital, in the sense of a through-space anomeric effect. This leads to a buildup of electron density in the intermonomer region and a distinct pnicogen bond strength order quantitatively assessed via k(a). However, the complex binding energy ΔE depends only partly on the pnicogen bond strength as H,E-attractions, H-bonding, dipole-dipole, or multipole-multipole attractions also contribute to the stability of pnicogen bonded dimers. A variation from through-space anomeric to second order hyperonjugative, and skewed π,π interactions is observed. Charge transfer into a π* substituent orbital of the acceptor increases the absolute value of ΔE by electrostatic effects but has a smaller impact on the pnicogen bond strength. A set of 10 dimers obtains its stability from covalent pnicogen bonding whereas all other dimers are stabilized by electrostatic interactions. The latter are quantified by the magnitude of the local intermonomer bending force constants XE···E′. Analysis of the frontier orbitals of monomer and dimer in connection with the investigation of electron difference densities, and atomic charges lead to a simple rationalization of the various facets of pnicogen bonding. The temperature at which a given dimer is observable under experimental conditions is provided.

Re‐evaluation of the bond length–bond strength rule: The stronger bond is not always the shorter bond

“…Often the argument is passed forward that the BDE values might describe at least qualitatively the correct trend of the intrinsic bond strength. However, numerous examples have been shown [35,[37][38][39]46,96] that this hope lacks any quantum mechanical basis and therefore it is always desirable to obtain the intrinsic strength of a bond from vibrational spectroscopy as the local stretching force constants can be always derived.…”

“…The preferred tool in this work will be a dynamical model of the bond strength based on vibrational spectroscopy and the theory of local modes, which was introduced by Konkoli and Cremer [28] and which, since then, has been proved to be physically sound, generally applicable, and a direct way of determining the intrinsic strength of a bond. [29][30][31] The usefulness of the local vibrational mode description of the bond strength has been documented by the characterization of CC bonds, [29,[32][33][34] NN bonds, [35] CO bonds, [36] CX bonds with X 5 F, Cl, Br, I, [37][38][39][40] H-bonding, [41][42][43][44] pnicogen bonding, [45,46] and the characterization of isotopomers. [47] The main objectives of this work are (i) to critically reevaluate previously published reverse BLBS relationships, (ii) to introduce reliable bond strength orders (BSOs) needed for characterizing unusual bonds, (iii) to elucidate the interplay of electronic and electrostatic factors determining the stability of electron-rich bonds, and (iv) to derive a rational for any deviation from the commonly known inverse BLBS relationships.…”

Quantitative Assessment of B−B−B, B−Hb−B, and B−Ht Bonds: From BH3 to B12H122−

“…It has been shown that the local stretching force constant k a is a superior bond strength measure compared to the often used bond dissociation energies (BDE) and bond dissociation enthalpies (BDH) as they contain cumulative effects of geometry relaxation, multiple bonding interactions, and also electronic density reorganizations of the dissociated fragments . Local stretching force constants k a have been successfully used to determine the intrinsic bond strength of covalent bonds such as: CC bonds, NN bonds, NF bonds, CO bonds, and CX (X=F, Cl, Br, I) bonds, and weak chemical interactions such as: hydrogen bonding, halogen bonding, pnicogen bonding, chalcogen bonding, tetrel bonding, and recently interactions …”

Tin···Oxygen Tetrel Bonding: A Combined Structural, Spectroscopic, and Computational Study

“…9,10,11,12,13,14 Over the past few years other non-conventional noncovalent interactions have gained attention, such as tetrel bonding, 15,16,17,18,19 pnictogen bonding, 20,21,22,23,24,25,26,27,28,29,30,31 chalcogenide bonding, 32,33,34,35,36,37,38,39,40,41,42,43,44 and even aerogen bonding. 45 All follow the same bonding scheme, namely a -hole, 46 which can be viewed comparably to hydrogen bonding: X-D … A, where X is any atom, D is the donor atom and A is the acceptor atom.…”