Nucleophilicity and electrophilicity of silylenes from a molecular electrostatic potential and dual descriptor perspectives José V. Correa a,b , Pablo Jaque a, * , Julianna Oláh c , Alejandro Toro-Labbé b , Paul Geerlings a a Eenheid Algemene Chemie, Faculteit Wetenschappen, Vrije Universiteit Brussel (VUB), Pleinlaan 2, Brussels 1050, Belgium b Laboratorio de Química Teórica Computacional (QTC), Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Correo 22, Santiago, Chile c School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK article info Article history: Received 24 December 2008 In final form 15 January 2009 Available online 21 February 2009 abstract Nucleophilicity and elecrophilicity of mono and disubstituted silylenes are analyzed using the molecular electrostatic potential and the dual descriptor Df ð ~ rÞ, defined within the so called conceptual DFT. A set of 32 compounds has been chosen which can be classified into 4 groups or families based on a linear rela- tionship between the molecular electrostatic potential measured in the electrophilic (empty 3p z orbital) V A and nucleophilic (lone pair) V min regions around of the silicon atom. The electrophilic and nucleophilic character of silylenes given by jV A  V min j is connected to the orbital-resolved dual descriptor Df 3p z through the p-electron donating property of the substituent. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Silylenes, RR 0 Si, are divalent silicon species analogous to carb- enes. Most of them are only known as reactive intermediates in thermal and photochemical reactions. Their existence has been proved by forming adducts with trapping agents while in recent years several stable species have been isolated [1–3]. In general, silylenes prefer a singlet configuration in their ground state. Sev- eral factors associated to the stabilization of singlet ground state silylenes are well-known, the p-electron donating ability from the substituent to the formally empty 3p z orbital of silicon being the most important one. Other factors are the aromaticity, and ring stress in the case of cyclic compounds [4–7]. The reactivity and stability patterns of silylenes have been stud- ied intensively, both experimentally and theoretically [1–10]. These aspects are strongly related: unstable silylenes show a strong electrophilic character whereas the stable species are highly nucleophilic. The most reactive center, in both cases, corresponds to the silicon atom. As can be seen from Scheme 1, singlet silylenes present two characteristic regions: region A corresponds to the empty 3p z orbital of the silicon atom, which is perpendicular to the molecular plane and responsible for the electrophilic character of these compounds. In region B a lone pair of electrons is localized in a r-orbital which accounts for the nucleophilic character of the singlet silylenes. It has been shown that substituents with p-elec- tron donating properties like –NH 2 , –SH, and –OH groups confer stability to the silicon center favouring the singlet electronic state. The particular situation that in silylenes electrophilic and nucle- ophilic regions are localized on the same atom, but in different directions incited various researchers to explore the merits of elec- trophilicity and nucleophilicity descriptors, traditionally used to differentiate between parts of molecular systems located on differ- ent atoms, to describe this ‘one center’ problem. In this context, some of the present authors have established a unified picture be- tween reactivity indices and stability for silylenes in their ground states as a function of the p-electron donating ability of the substi- tuent (see Scheme 4 in Ref. [11]). Density Functional Theory (DFT)- based reactivity descriptors were used in conjunction with the molecular electrostatic potential (V ð ~ rÞ) as measures of chemical reactivity, whereas the reaction enthalpy for isodesmic reaction (RR 0 Si + SiH 4 ! RR 0 SiH 2 +H 2 Si), the dimerization energy (RR 0 Si + RR 0 Si ! RR 0 Si@SiRR 0 ), and the singlet–triplet energy difference were used as measures of stability. An interesting result from this study is a connection of electro- philicity and nucleophilicity with the 3p z electron population of the silicon atom governed by the p-donating character of the sub- stitutents: increasing p-electron donating capability of the substit- uents increases the population of the 3p z orbital of the Si atom which is reflected in a less positive electrostatic potential in region A yielding a lower electrophilicity. On the other hand, a diminish- ing electrophilic power is accompanied by an increasing nucleo- philicity in the lone pair region possibly associated with an electron delocalization effect between the 3p z and r orbitals of Si atom. In the present Letter we elaborate on this remarkable property of silylenes probing the different reactivity in different directions on a single center through a combined use of the molecular elec- trostatic potential and conceptual DFT based descriptors. 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.01.054 * Corresponding author. Fax: +32 2 629 3317. E-mail addresses: pjaqueol@vub.ac.be (P. Jaque), pgeerlin@vub.ac.be (P. Geer- lings). Chemical Physics Letters 470 (2009) 180–186 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett