Three groups of singlet ground state [TCNE]22−(TCNE=tetracyanoethylene) dimers with characteristic intradimer CC separations (r) and dihedral angles (d) [i.e., group St(r∼1.6 Å; d=180°), Lt(r∼3.5 Å; d=180°), and Lc(r∼2.9 Å; d=∼0°); notation: S/L: short/long bond length; subscript t/c: trans/cis, respectively] are experimentally characterized. The Stgroup is comprised of σ‐dimers of [TCNE].−and octacyanobutanediide, [C4(CN)8]2−, which have a typical, albeit long, sp3–sp3σ bond (r∼1.6 Å) between each [TCNE].−moiety and characteristic νCN, νCC, and δCCNIR absorptions. The L groups are structurally characterized as π‐dimers of [TCNE].−that are either eclipsed with r∼2.9 Å (Lc) and the nitriles bend away from the nominal TCNE plane away from the center of the dimer by 5.0° (∼sp2.17) or are noneclipsed with r∼3.5 Å (Lt) and the nitriles bend toward the center of the dimer by 1.9° (∼sp2.06). Ab initio computations on isolated dimers were used to study the formation and stability of these exceptionally long CC (≥2.9 Å) bonding interactions as well as the process of π‐[TCNE]22−dimer formation for the Lcand Ltgroups. The results of these computational studies show that the ground‐state potential curve is that of a closed‐shell/open‐shell singlet, depending on the distance. The short Stgroup (r∼1.6 Å) of dimers in this surface are true minimum‐energy structures; however, the Ltand Lcgroups are unstable, although two different nonphysical minima are found when imposing a double occupancy of the orbitals. These minima are metastable relative to dissociation into the isolated [TCNE].−units. Consequently, the existence of dimer dianions in crystals is due to cation⋅⋅⋅[TCNE]−interactions, which provide the electrostatic stabilization necessary to overcome the intradimer electrostatic repulsion. This cation‐mediated π*–π* [TCNE]−⋅⋅⋅[TCNE]−interaction complies with Pauling's definition of a chemical bond. This bonding interaction involves the π* orbitals of each fragment, and arise from the overlap of the b2gSOMO on each of the two [TCNE].−s to form a filled b2u[TCNE]22−orbital. Although a π dimer typically forms, if the fragments are close enough a σ dimer can form. Due to the presence of cation‐mediated intradimer CC bonding interactions the Lcgroup of π‐[TCNE]22−dimers exhibits experimentally observable νCNIR absorptions at 2191±2 (m), 2173±3 (s), and 2162±3 cm−1(s) and νCCat 1364±3 cm−1(s) as well as a new UV‐Vis feature in the range of 15 000 to 18 200 cm−1(549 to 667 nm) and averaging 16 825±1180 cm−1(594 nm) assigned to the predicted new intradimer 1A1g→ 1B1utransition and is purple on reflected light. Upon cooling to 77 K in 2‐methyl tetrahydrofuran, this new band occurs at 18 940 cm−1(528 nm) for {[Et4N]+}2[TCNE]22−, and the yellow solution turns deep red. Group Ltis characterized by νCNabsorptions at 2215±2, 2197±3, and 2180±4 cm−1and νCCat 1209±9 cm−1(w), while group SThas νCNbands at 2215±4, 2157±3, and 2107±4 cm−1and νCCat 1385±1 cm−1(vs).