Three groups of singlet ground state TCNE (2-) (TCNE=tetracyanoethylene) dimers with characteristic intradimer CC separations (r) and dihedral angles (d) [i.e., group S(t) (r approximately 1.6 A; d=180 degrees ), L(t) (r approximately 3.5 A; d=180 degrees ), and L(c) (r approximately 2.9 A; d= approximately 0 degrees ); notation: S/L: short/long bond length; subscript t/c: trans/cis, respectively] are experimentally characterized. The S(t) group is comprised of sigma-dimers of TCNE and octacyanobutanediide, C(4)(CN)(8), which have a typical, albeit long, sp(3)-sp(3) sigma bond (r approximately 1.6 A) between each TCNE moiety and characteristic nu(CN), nu(CC), and delta(CCN) IR absorptions. The L groups are structurally characterized as pi-dimers of TCNE that are either eclipsed with r approximately 2.9 A (L(c)) and the nitriles bend away from the nominal TCNE plane away from the center of the dimer by 5.0 degrees (approximately sp(2.17)) or are noneclipsed with r approximately 3.5 A (L(t)) and the nitriles bend toward the center of the dimer by 1.9 degrees ( approximately sp(2.06)). Ab initio computations on isolated dimers were used to study the formation and stability of these exceptionally long CC (> or =2.9 A) bonding interactions as well as the process of pi-TCNE (2-) dimer formation for the L(c) and L(t) groups. 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 S(t) group (r approximately 1.6 A) of dimers in this surface are true minimum-energy structures; however, the L(t) and L(c) groups 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 pi*-pi* TCNE.TCNE interaction complies with Pauling's definition of a chemical bond. This bonding interaction involves the pi* orbitals of each fragment, and arise from the overlap of the b(2g) SOMO on each of the two TCNEs to form a filled b(2u) TCNE (2-) orbital. Although a pi dimer typically forms, if the fragments are close enough a sigma dimer can form. Due to the presence of cation-mediated intradimer CC bonding interactions the L(c) group of pi-TCNE (2-) dimers exhibits experimentally observable nu(CN) IR absorptions at 2191+/-2 (m), 2173+/-3 (s), and 2162+/-3 cm(-1) (s) and nu(CC) at 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 (1)A(1g) --> (1)B(1u) transition 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 [Et(4)N](2)TCNE (2-), and the yellow solution turns deep red. Group L(t) is characterized by nu(CN) absorptions at 2215+/-2, 2197+/-3, and 2180+/-4 cm(-1) and nu(CC) at 1209+/-9 cm(-1) (w), while group S(T) has nu(CN) bands at 2215+/-4, 2157+/-3, and 2107+/-4 cm(-1) and nu(CC) at 1385+/-1 cm(-1) (vs).