A New Conformation With an Extraordinarily Long, 3.04 Å Two‐Electron, Six‐Center Bond Observed for the π‐[TCNE] 2 2− Dimer in [NMe 4 ] 2 [TCNE] 2 (TCNE=Tetracyanoethylene)

[NMe 4 ] 2 [TCNE] 2 (TCNE=tetracyanoethenide) formed from the reaction of TCNE and (NMe 4 )CN in MeCN has ν CN IR absorptions at 2195, 2191, 2172, and 2156 cm −1 and a ν CC absorption at 1383 cm −1 that are characteristic of reduced TCNE. The TCNEs have an average central CC distance of 1.423 Å that is also characteristic of reduced TCNE. The reduced TCNE forms a previously unknown non‐eclipsed, centrosymmetric π‐[TCNE] 2 2− dimer with nominal C 2 symmetry, 12 sub van der Waals interatomic contacts <3.3 Å, a central intradimer separation of 3.039(3) Å, and comparable intradimer C⋅⋅⋅N distances of 3.050(3) and 2.984(3) Å. The two pairs of central C⋅⋅⋅C atoms form a ∢CC⋅⋅⋅CC of 112.6° that is substantially greater than the 0° observed for the eclipsed D 2 h π‐[TCNE] 2 2− dimer possessing a two‐electron, four‐center (2e − /4c) bond with two C⋅⋅⋅C components from a molecular orbital (MO) analysis. A MO study combining CAS(2,2)/MRMP2/cc‐pVTZ and atoms‐in‐molecules (AIM) calculations indicates that the non‐eclipsed, C 2 π‐[TCNE] 2 2− dimer exhibits a new type of a long, intradimer bond involving one strong C⋅⋅⋅C and two weak C⋅⋅⋅N components, that is, a 2e − /6c bond. The C 2 π‐[TCNE] 2 2− conformer has a singlet, diamagnetic ground state with a thermally populated triplet excited state with J / k B =1000 K (700 cm −1 ; 86.8 meV; 2.00 kcal mol −1 ; H =−2  J S a ⋅S b ); at the CAS(2,2)/MBMP2 level the triplet is computed to be 9.0 kcal mol −1 higher in energy than the closed‐shell singlet ground state. The results from CAS(2,2)/NEVPT2/cc‐pVTZ calculations indicate that the C 2 and D 2 h conformers have two different local metastable minima with the C 2 conformer being 1.3 kcal mol −1 less stable. The different natures of the C 2 and D 2 h conformers are also noted from the results of valence bond (VB) qualitative diagram that shows a 10e − /6c bond with one C⋅⋅⋅C and two C⋅⋅⋅N bonding components for the C 2 conformer as compared to the 6e − /4c bond for the D 2 h conformer with two C⋅⋅⋅C bonding components.