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 <i>ab initio</i> 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