Four-center carbon-carbon bonding.

The 2.89-A intradimer separation for cofacial [TCNE]2(2-) (TCNE = tetracyanoethylene) dimers is twice that of conventional C-C bonds but approximately 0.6 A shorter than the sum of the van der Waals radii. Experimental and computational studies best characterize the intradimer bonding as a 2-electron-4-center (2e(-)-4c) C-C bonding interaction (or bond). This nonconventional bonding exhibits unique spectroscopic properties (new, lower energy electronic absorption, new nuCC and new and shifted nuCN and deltaCCN vibrational absorptions, and characteristic 13C NMR chemical shifts) and is diamagnetic. [TCNE]2(2-) is a prototype of a growing number of organic compounds that are best described by multicenter C-C bonding.