title: Multicenter Bonding in Ditetracyanoethylene Dianion: A Simple Aromatic Picture in Terms of Three-Electron Bonds.
abstract: The nature of the multicenter, long bond in ditetracyanoethylene dianion complex [TCNE]2(2-) is elucidated using high level ab initio Valence Bond (VB) theory coupled with Quantum Monte Carlo (QMC) methods. This dimer is the prototype of the general family of pancake-bonded dimers with large interplanar separations. Quantitative results obtained with a compact wave function in terms of only six VB structures match the reference CCSD(T) bonding energies. Analysis of the VB wave function shows that the weights of the VB structures are not compatible with a covalent bond between the π* orbitals of the fragments. On the other hand, these weights are consistent with a simple picture in terms of two resonating bonding schemes, one displaying a pair of interfragment three-electron σ bonds and the other displaying intrafragment three-electron π bonds. This simple picture explains at once (1) the long interfragment bond length, which is independent of the countercations but typical of three-electron (3-e) CC σ bonds, (2) the interfragment orbital overlaps which are very close to the theoretical optimal overlap of 1/6 for a 3-e σ bond, and (3) the unusual importance of dynamic correlation, which is precisely the main bonding component of 3-e bonds. Moreover, it is shown that the [TCNE]2(2-) system is topologically equivalent to the square C4H4(2-) dianion, a well-established aromatic system. To better understand the role of the cyano substituents, the unsubstituted diethylenic Na(+)2[C2H4]2(2-) complex is studied and shown to be only metastable and topologically equivalent to a rectangular C4H4(2-) dianion, devoid of aromaticity.
authors.affiliation: UPMC Université Paris 06, CNRS UMR 7616, Laboratoire de Chimie Théorique, C. 137, 4 Place Jussieu, 75252 Paris Cedex 05, France.
authors.forename: Benoit
authors.initials: B
authors.lastname: Braida
authors.affiliation: UPMC Université Paris 06, CNRS UMR 7616, Laboratoire de Chimie Théorique, C. 137, 4 Place Jussieu, 75252 Paris Cedex 05, France.
authors.forename: Kevin
authors.initials: K
authors.lastname: Hendrickx
authors.affiliation: UPMC Université Paris 06, CNRS UMR 7616, Laboratoire de Chimie Théorique, C. 137, 4 Place Jussieu, 75252 Paris Cedex 05, France.
authors.forename: Dominik
authors.initials: D
authors.lastname: Domin
authors.affiliation: University of Rochester, Department of Chemistry, Rochester, New York 14627, United States.
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Dinnocenzo
authors.affiliation: Laboratoire de Chimie Physique, UMR CNRS 8000, Groupe de Chimie Théorique, Université de Paris-Sud, 91405 Orsay Cédex, France.
authors.forename: Philippe C
authors.initials: PC
authors.lastname: Hiberty