[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]22− 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 νC≡N absorptions at 2191, 2174, 2169, 2163 and a νCC 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]22− interactions do not provide evidence that the cation stabilizes formation of the [TCNE]22− dimer, which is stabilized by the intradimer 2e−-4 center C–C bonding interaction.