Three-Electron Two-Center Bonding in Cycloimmonium Ylides P. KARAFILOGLOU, 1 G. SURPATEANU 2 1 Faculty of Chemistry, Department of General and Inorganic Chemistry, POB 135, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece 2 Laboratoire de Synthe `se Organique et Environnement, Universite ´ du Littoral, Co ˆte d’Opale 145, Av. M. Schumann, 59140 Dunkerque, France Received 15 September 2003; accepted 10 November 2003 Published online 8 March 2004 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/qua.20010 ABSTRACT: We explore the possibility that a 3-electron-2-center bonding exists in cycloimmonium ylides. To detect this bonding in a polyatomic system, 3-electron-1-hole density operators, characterizing a Pauling 3-electron bond, are used in the framework of second quantization formalism. The weights of 3-electron resonance structures are calculated and compared with the weights of 2-electron structures for the ylide bond of pyridinium dicyanomethylide; the correlations of (12) and (1) electronic events, involved in the 3-electron resonance structures, are also investigated. The calculations are performed in various approximation levels, and both orthogonal and nonorthogonal natural atomic orbitals are adopted. All calculations show that a 3-electron bond exists between N and C atoms of ylide bond, but this bonding is not extended in C atoms of the pyridinium group. The interactions of , electrons (at the configuration interaction [CI] level) increase the localization of electrons, the weights of 3-electron resonance structures, and thus the probability for 3-electron bonding. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem 98: 456 – 464, 2004 Key words: three-electron bonds; cycloimmonium ylides; density operators; electronic fluctuations; resonance structures; population analysis Introduction C ycloimmonium ylides [1, 2], obtained from various synthetic methods, form a well- known class of zwitteronic compounds that display interesting chemical behavior [3–5]. A representa- tive compound of this class of ylides is the pyri- dinium dicyanomethylide (PDCM), shown in Fig- ure 1. Although this type of ylide is investigated from various experimental and theoretical points of view [4, 5], the possibility of 3-electron-2-center bonding has not been explored. Admittedly, traditional chemical formula gener- ally plays a privileged role in all quantum chemical interpretations and theories [6]. In this context, Pauling’s “resonance” (or “mesomeric”) theory can provide quite intuitive and chemically meaningful Correspondence to: P. Karafiloglou; e-mail: karafilo@chem.auth.gr International Journal of Quantum Chemistry, Vol 98, 456 – 464 (2004) © 2004 Wiley Periodicals, Inc.