Theoretical analysis of the electronic structure and bonding stability of the TCNE dimer dianion (TCNE) 2 2-.

The (TCNE)(2)(2)(-) dimer dianion formed by connecting two TCNE(-) anions via a four-center, two-electron pi-orbital bond is studied using ab initio theoretical methods and a model designed to simulate the stabilization due to surrounding counterions. (TCNE)(2)(2)(-) is examined as an isolated species and in a solvation environment representative of tetrahydrofuran (THF) solvent. The intrinsic strength of this novel bond and the influences of internal Coulomb repulsions, of solvent stabilization and screening, and of counterion stabilization are all considered. The geometry, electronic and thermodynamic stabilities, electronic absorption spectra, and electron detachment energies of this novel dianion are examined to help understand recent experimental findings. Our findings lead us to conclude that the (TCNE)(2)(2)(-) dianion's observation in solid materials is likely a result of its stabilization by surrounding countercations. Moreover, our results suggest the dianion is geometrically metastable in THF solution, with a barrier to dissociation into two TCNE(-) anions that can be quickly surmounted at room temperature but not at 77 K. This finding is consistent with what is observed in laboratory studies of low- and room-temperature solutions of salts containing this dianion. Finally, we assign two peaks observed (at 77 K in methyl-THF glass) in the UV-vis region to (1) electronic transitions involving the four-center orbitals and (2) detachment of an electron from the four-center pi-bonding orbital to generate (TCNE)(2)(-) + e(-).