[MeNC5H5]2[TCNE]2 (TCNE = tetracyanoethylene). Single crystal X-ray and neutron diffraction characterization of an exceptionally long 2.8 Å C-C bond

The reaction of N-methylpyridinium iodide, Mepy+I-, and tetracyanoethylene (TCNE) forms [Mepy]2[TCNE]2, which possesses [TCNE]2 2' with an intradimer C-C bond distance of 2.806(1)Å at 50 K from X-ray diffraction, and 2.801(4)Å at 50 K from neutron diffraction. In the IR it exhibits nChN absorptions at 2191, 2174, 2169, 2163 and a nCC absorption at 1366 cm-1, with UV/Vis absorption bands at 26,880 and 18,520 cm-1. Analysis of the cation-hydrogen to [TCNE]2 2- interactions do not provide evidence that the cation stabilizes formation of the [TCNE]2 2- dimer, which is stabilized by the intradimer 2e--4 center C-C bonding interaction.