Charge Density Study of Two-Electron Four-Center Bonding in a Dimer of Tetracyanoethylene Radical Anions as a...

The dimer of the tetracyanoethylene (TCNE) radical anions represents the simplest and the best studied case of two-electron multicenter covalent bonding (2e/mc or pancake bonding). The model compound, N-methylpyridinium salt of TCNE^•-, is diamagnetic, meaning that the electrons in two contiguous radicals are paired and occupy a HOMO orbital which spans two TCNE^•- radicals. Charge density in this system is studied as a benchmark for comparison of charge densities in other pancake-bonded radical systems. Two electrons from two contiguous radicals indeed form a bonding electron pair, which is distributed between two central ethylene groups in the dimer, i.e., between four carbon atoms. The topology of electron density reveals two bond critical points between the central ethylene groups in the dimer, with maximum electron density of 0.185 e Å^-3; the corresponding theoretical value is 0.118 e Å^-3.