Electron-Rich Three-Center Bonding:  Role of s,p Interactions across the p-Block

Abstract

This paper analyzes the importance of s,p mixing-a necessary addition to the simplest Rundle-Pimentel picture-and periodic and group trends in electron-rich three-center bonding. Our analysis proceeds through a detailed quantum chemical study of the stability of electron-rich three-center bonding in triatomic 22-valence electron anions. To provide interpretations, a perturbational molecular orbital (MO) analysis of s,p mixing is carried out. This analysis of the orbitals and the overlap populations is then tested by density functional calculations for a number of linear trihalides, trichalcogenides, and tripnictides. The most important effect of s,p mixing on the in-line bonding is in destabilization of the 3sigma(g) orbital and is determined by the overlap between the s orbital of the central atom and the p orbital of the terminal atom. Further destabilization arises from the repulsion of p(pi) lone pairs. Both of these antibonding effects increase with increasing negative charge of the system. The stability of isoelectronic X(3) systems thus decreases when moving from right to left in the periodic table. Interesting group trends are discerned; for instance, for the electron-rich tripnictides, the ability to accommodate a hypervalent electron count is the largest in the middle rather at the end of the group. Particularly strong s,p mixing can reverse the bonding/antibonding character of MOs: thus MO 2sigma(u) that is responsible for bonding for trihalides and trichalcogenides is actually antibonding in N(3)(7)(-).

Charge transfer complexes of dithioxamides with dihalogens as powerful reagents in the dissolution of noble metals

“…Accordingly, the system must be described by a model that changes dynamically with limiting forms. Recently, Hoffman et al [13] have illustrated the electronic effects for I 3 − and for the anions of the type X-I-I − (X = halogen) by applying the concepts of perturbation theory to three-centre systems. Essentially the original three-centre/fourelectron model for the bonding proposed by Pimentel and Rundle is confirmed [14].…”

cis‐Bromierung von Alkinen ohne kationische Zwischenstufen

“…Mithilfe des einfachen VB-Modells interpretiert, ist das Tribromid-Ion eine lineare 22-Elektronen-Spezies mit einem trigonal-planaren sp 3 d-hybridisierten zentralen Br-Atom, dessen drei freie Elektronenpaare die äquatorialen Positionen besetzen; die Wechselwirkung eines der freien Elektronenpaare mit einem Kation führt daher zu einem T-förmigen Molekül. Trotz seiner ungewöhnlichen Struktur [21] [23][24][25] zeigen, dass die Aktivierungsbarriere (7!9)°für die Bildung des Aktivierungsenergie zur Bildung von 15 aus dem p-Komplex 14 zumindest im Stammsystem Ethylen zu hoch, um mit dem klassischen Mechanismus über das Bromonium-Ion zu konkurrieren. In substituierten Systemen wie Acenaphthylen, das unter geeigneten Reaktionsbedingungen Brom vorzugsweise cis-addiert, [7,8] …”

cis‐Bromination of Alkynes without Cationic Intermediates

“…The interaction of one of the lone pairs with a cation therefore generates a Tshaped molecule. Despite its unusual structure [21] the tribromide adduct 10 is more than 10 kcal mol À1 more stable than the reactants and is a well-defined minimum (also at the MP2/ 6-31G* level of theory). The dibromo adduct 11 is formed from 10 by a "shift" of the Br 3 À unit and elimination of Br 2 .…”

“…In substituted systems like acenaphthylene, which adds bromine preferentially cis, however, an intermediate analogous to 15 is likely. [7,8] Received: August 13, 2004 Revised: November 3, 2004 Published online: January 21,2005 . Keywords: alkynes · bromination · density functional calculations · electrophilic addition · reaction mechanisms…”