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 similar to 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 nu(CC) and new and shifted nu(CN) and delta(CCN) vibrational absorptions, and characteristic C-13 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.